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The  Electromotive  Force  of  Iron  Under 
Varying  Conditions,  and  the  Effect 

of  Occluded  Hydrogen 


BY 

THEODORE  WILLIAM  RICHARDS 

AND 

GUSTAVUS  EDWARD  BEHR,  JR. 


PUBLISHED  BY  THE 

CARNEGIE  INSTITUTION  OF  WASHINGTON 

1906 


V  \ 


UNIVERSITY  OF  CALIFORNIA 
DAVIS 


CARNEGIE  INSTITUTION  OF  WASHINGTON,  PUBLICATION  No.  6i 


CONTRIBUTIONS  FROM  THE  CHEMICAL  LABORATORY 
OF  HARVARD  COLLEGE 


THE  KRIEDENWALD  COMPANY 
BALTIMORE,  MD.,  U.  8.  A- 


THE  ELECTROMOTIVE  FORCE  OF  IRON  UNDER  VARYING 

CONDITIONS,  AND  THE  EFFECT  OF 

OCCLUDED  HYDROGEN. 


The  topics  named  in  the  title  of  this  paper  are  of  great  practical  import- 
ance, as  well  as  of  great  interest  from  the  theoretical  point  of  view.  The 
rate  of  rusting  and  the  change  of  properties  caused  by  the  occlusion  of 
hydrogen  are  essential  factors  in  determining  the  permanence  and  strength 
of  most  modern  buildings,  bridges,  and  underground  pipes.  For  this  reason 
the  literature  which  has  already  accumulated  upon  the  subject  has  grown 
almost  to  the  dimensions  of  a  library  in  itself.  The  theoretical  side  of 
the  matter  also  is  of  great  interest.  The  cause  of  the  sudden  diminution 
of  activity  produced  in  iron  under  certain  well-known  circumstances  has  not 
yet  found  its  definitive  explanation,  and  the  mechanism  of  the  occlusion  of 
hydrogen  by  a  solid  so  tenacious  and  compact  as  iron  is  a  theoretical  ques- 
tion of  no  small  interest. 

Phenomena  relating  to  these  topics  are  considered  together  in  this  paper, 
because  the  method  of  investigation  furnished  evidence  with  regard  to  both. 
The  matter  was  considered  rather  from  the  theoretical  than  the  practical 
standpoint.  The  method  was  the  determination  of  the  electromotive  force 
of  iron,  after  subjection  to  a  great  variety  of  changing  conditions,  on  immer- 
sion in  a  solution  of  one  of  its  salts,  in  order  by  comparison  of  these  different 
values  of  electromotive  force  to  draw  inference  concerning  the  state  of  the 
metal  and  its  occluded  gas. 

The  object  of  the  research,  as  originally  planned,  was  to  add  to  the  experi- 
mental evidence  concerning  the  significance  of  changing  atomic  volume,  and 
it  was  proposed  to  discover  whether  or  not  changes  in  free  energy  were 
associated  with  changes  of  density  and  of  other  properties  of  the  solid. 
During  the  progress  of  the  work  it  was  found  that  this  matter  could  not  be 
satisfactorily  studied  without  studying  the  properties  of  occluded  hydrogen, 
because,  in  spite  of  the  great  volume  of  literature  upon  the  subject,  certain 
important  questions  seemed  to  have  remained  hitherto  almost  untouched. 
Lack  of  time  has  as  yet  prevented  the  completion  of  the  program ;  but  the 

3 


4  ELECTROMOTIVE   FORCE  OF   IRON   AND   OCCLUDED   HYDROGEN. 

present  results,  although  only  a  part  of  the  whole  plan,  are  reasonably  com- 
plete in  themselves,  and  therefore  worthy  of  separate  publication.  The 
subject  will  be  taken  up  again  in  the  near  future. 

For  the  sake  of  clearness  and  convenience,  the  present  paper  will  be 
divided  into  two  parts,  entitled :  Part  I.  "  The  Electromotive  Force  of  Pure 
Iron  under  Varying  Conditions ; "  Part  II.  "  The  Electromotive  Force  of 
Iron  Containing  Occluded  Hydrogen." 


PART  I.    THE  ELECTROMOTIVE  FORCE  OF  PURE  IRON 
UNDER  VARYING  CONDITIONS. 


When  in  a  definite  state  of  internal  equilibrium,  the  electromotive  force  of 
a  metal  immersed  as  a  reversible  electrode  in  a  definite  solution  of  one  of  its 
salts  must  have  a  perfectly  definite  value  under  given  conditions.  A  change 
in  the  internal  state  must  of  necessity  change  this  electromotive  force,  unless 
the  temperature  happens  to  be  exactly  at  the  transition  point.  For  this 
reason  it  seemed  not  improbable  that  the  measurement  of  the  electromotive 
force  of  iron  which  had  been  subjected  to  great  stress  would  afford  the  most 
convenient  method  of  determining  whether  or  not  this  stress  had  affected  a 
permanent  alteration  in  the  internal  equilibrium  of  the  metal,  a  "  permanent 
set,"  as  it  is  sometimes  called,  or  an  extraordinary  condition  of  metastable 
equilibrium. 

A  solid  may  be  subjected  to  a  variety  of  stresses.  The  first  which  it 
seemed  desirable  to  study  was  direct  pressure,  in  order  to  determine  if  this 
pressure,  by  causing  closer  internal  structure,  diminishes  the  outward  or 
dissolving  tendency  as  measured  by  electromotive  force,  or  if,  on  the  other 
hand,  the  substance,  when  released  from  the  pressure,  returns  at  once  to 
essentially  the  state  in  which  it  was  before  the  pressure  was  applied.  Experi- 
ment alone  could  decide  between  these  alternatives. 

It  was  desirable  also  to  test  the  possibly  opposite  effect  of  a  negative 
pressure,  a  distending  tendency,  such  as  the  effect  of  stretching  a  wire  to 
the  breaking-point.  This  effect  was  also  made  the  subject  of  experiments 
recorded  below. 

During  these  tests  it  was  found  that  the  temperature  used  in  the  reduction 
of  the  iron  from  its  oxide  caused  a  very  important  effect  on  the  dissolving 
tendency,  and  therefore  this  matter  also  had  to  be  considered  in  a  special 
series  of  experiments. 

PREPARATION  OF  MATERIAL. 

In  any  research  with  iron  it  is  of  primary  importance  that  the  substance 
experimented  upon  should  be  pure — free  from  carbon,  silicon,  sulphur, 
phosphorus,  manganese,  and,  as  the  following  work  testifies,  from  hydrogen 
also. 

5 


b  ELECTROMOTIVE   FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 

Most  of  the  iron  used  in  this  research  was  prepared  by  a  method  which 
had  been  found  in  an  exhaustive  investigation  of  the  atomic  weight  of  the 
metal  to  yield  good  results.^  It  is  true  that  not  quite  all  the  steps  were  used 
in  the  present  work,  but  the  more  essential  ones  were  retained,  and  a  very 
pure  sample  of  the  metal  was  prepared.  "  Piano "  wire  of  good  quality 
served  as  the  original  substance ;  it  was  carefully  sandpapered  and  wiped, 
and  dissolved  in  dilute  nitric  acid.  Concentrated  acid  was  first  used,  but  this 
oxidized  the  silicon  to  silicic  acid,  and  filtration  from  the  silicon  itself  (or 
silicide  of  iron,  SiFcg)/  which  the  dilute  acid  leaves  unchanged,  was  con- 
sidered more  safe,  besides  being  more  convenient.  All  iron  carbide  or 
graphite  is  either  completely  oxidized  or  else  is  unchanged  and  filtered  out. 
The  dilute  acid  also  lessens  the  chances  of  any  sulphur  being  oxidized  to 
sulphate. 

The  nitrate  formed  was  then  concentrated  and  recrystallized  three  or  four 
times.  Recrystallization  is  here  a  peculiarly  good  method  of  purification, 
because  the  ferric  nitrate  has  an  unusual  crystal  form ;  therefore  most  im- 
purities can  only  be  held  mechanically.'  The  recrystallized  salt  was  then 
heated  in  a  platinum  dish  at  a  moderate  temperature  in  an  electric  oven. 
The  nitric  acid  was  partly  driven  off  and  the  dry,  dark-red  basic  nitrate  was 
then  powdered  in  a  Hempel  steel  mortar*  and  heated  to  full  redness  to 
complete  the  denitration.  It  is  important  that  this  operation  be  carried  out 
in  steps  as  described,  because  in  this  way  a  fine,  spongy  powder  is  obtained 
which  allows  of  perfect  reduction.  If  denitrated  too  quickly  by  too  high  a 
temperature  the  oxide  obtained  is  exceedingly  hard  and  the  powder  con- 
sists of  very  compact  particles,  in  which  the  complete  reduction  of  the  core 
is  more  or  less  uncertain  and  decidedly  less  rapid. 

The  oxide  so  produced  was  reduced  in  a  stream  of  electrolytic  hydrogen ' 
in  an  unglazed  porcelain  boat  °  set  in  a  hard  glass  combustion-tube  when  a 
low-temperature  reduction  was  desired,  or  in  a  large  glazed  porcelain  tube  ^ 
in  a  gas-furnace  when  a  higher  temperature  was  used. 


*  Richards  and  Baxter,  Z.  f.  anorg.  ch.,  23,  245  (1900)  ;  Proc.  Amer.  Acad.,  35,  253 
(1900). 

^Lebeau,  Bull.  Soc.  Chim.,  Paris,  (3)  27,  39-42  (1902);  Ann.  Chim.  Phys.  (7),  26, 
5-31  (1902). 

'T.  W.  Richards,  Zeitschr.  f.  phys.  Chem.,  46,  189  (1903). 

*W.  Hempel,  Z.  f.  ang.  Chem.  14,  843  (1901). 

"The  generators  used  were  zinc  amalgam,  dilute  hydrochloric  acid,  platinum  cells, 
and  were  of  the  convenient  form  used  by  Cooke  and  Richards,  Proc.  Amer.  Acad.  23, 
168  (1887).  The  hydrogen  was  twice  washed  in  strong  caustic  solution  and  dried 
by  passing  through  two  caustic  potash  towers. 

*  Unglazed,  because  at  the  high  temperature  used  the  glaze  frequently  fused  with 
the  iron. 

*  Hempel  water-cooled  stoppers  were  used  in  the  porcelain  tube. 


THE  REDUCTION   OF   PURE   IRON.  7 

The  iron  from  these  reductions  was  of  a  gray  color  and  of  a  porous  struc- 
ture. A  piece  of  it  could  usually  be  broken  with  ease  between  the  fingers 
and  rubbed  to  powder.  This  consistency  was  of  course  due  to  its  mode  of 
preparation,  the  powdered  oxide  having  been  simply  pressed  into  the  boats 
and  the  temperature  having  been  only  high  enough  to  cause  slight  cohesion 
between  the  particles,  not  to  fuse  the  mass. 

The  cohesion  of  the  particles  varied  considerably  with  the  temperature  at 
which  the  reduction  was  carried  on,  a  product  reduced  at  600°  having  no 
cohesion  at  all,  while  one  heated  to  about  1,100°  for  some  time  could  be 
broken  only  with  some  difficulty.  The  color  also  varied,  owing  to  the  differ- 
ence in  degree  of  subdivision,  from  almost  black  to  a  clear,  silvery  gray. 

A  convenient  index  of  complete  reduction  was  the  absence  of  any  trace 
of  black  oxide  in  the  cooler  portions  of  the  boat;  for  water  formed  in  the 
hotter  places  is  decomposed  in  the  somewhat  cooler  ones,  and  if  the  reduc- 
tion is  not  complete  water  is  of  course  present.  Ordinarily,  the  reduction 
was  carried  out  by  heating  the  substance  in  a  rapid  stream  of  hydrogen  for 
three  or  four  hours  with  a  Bunsen  burner,  and  then  for  about  half  an  hour 
at  the  highest  temperature  of  the  blast  lamp.  Such  iron  showed  not  the 
slightest  trace  of  black  oxide,  even  when  finely  powdered  in  a  mortar.  After 
heating  the  iron  remained  in  a  continuous  stream  of  dry  hydrogen  until 
quite  cold,  and  was  then  transferred  for  preservation  to  a  good  desiccator 
containing  finely  divided  fused  caustic  potash.  No  trace  of  oxidation  was 
ever  visible  on  iron  thus  preserved;  but,  as  will  be  explained  later,  the 
electromotive  force  measurements  gave  clear  evidence  that  an  exceedingly 
thin  coating  of  oxide  or  adsorbed  oxygen  had  been  formed,  and  therefore, 
for  the  more  accurate  experiments,  small  boat-loads  of  pieces  about  the  size 
of  peas  were  reduced  a  second  time  and  sealed  in  hydrogen  until  just  before 
using,  thus  avoiding  the  possibility  of  oxidation.  Whenever  this  specially 
sealed  iron  is  used  in  the  experiments  below  the  fact  is  mentioned.  The 
sealing  was  done  as  follows:  A  hard  glass  tube  was  drawn  out  in  several 
places,  having  between  each  contracted  place  a  boat  containing  porous  iron. 
Three  or  four  such  boats  in  series  were  laid  in  a  combustion  furnace,  and 
after  reduction  the  boats  were  sealed  off  by  drawing  out  the  contracted  places. 

All  this  iron  undoubtedly  contained  hydrogen  in  greater  or  less  amounts. 
Although,  as  Baxter'  has  shown,  the  weight  of  hydrogen  retained  by  pure 
iron  ignited  at  a  high  temperature  is  inconsiderable,  some  of  the  other 
properties  of  the  occluded  gas  are  more  important,  as  will  be  seen.  Most 
of  the  hydrogen  can  be  expelled  by  heating  for  a  long  time  in  vacuum,  or 
by  standing  for  a  long  time  in  dry  air;  but  in  order  to  drive  out  the  last 
traces  fusion  in  vacuum  is  necessary. 


"Baxter,  Am.  Chem.  Journ.,  22,  363  (1899). 


ELECTROMOTIVE  FORCE  OF   IRON   AND  OCCLUDED   HYDROGEN. 


METHOD  OF  MEASUREMENT. 

The  solution-tension  of  the  iron  was  measured  as  its  electromotive  force 
in  a  normal  solution  of  ferrous  salt,  in  this  case  ferrous  sulphate.  The 
sulphate  ion  is  desirable  here,  since  it  is  neutral  as  regards  passivity  phe- 
nomena, having-  neither  passivizing  nor  activizing  effect."  The  voltage  of 
the  cell  Fe,  nFeSOi,  n/ioKCl,  HgCl,  Hg,  was  measured  by  the  well- 
known  compensation  method  of  Poggendorff." 

A  d'Arsonval  galvanometer  served  as  zero  instrument,  being  sensitive  to 
o.ooi  volt  with  the  resistance  used.  A  Helmholtz  cell  was  used  as  a  standard 
of  comparison.  Its  exact  value  was  0.993  international  volt,  as  determined 
by  a  comparison  with  carefully  prepared  cadmium  normal  elements.  These 
agreed  accurately  among  themselves,  and  were  assumed  to  possess  at 
21.50°  C.  the  value  1.0185  volts.  They  agreed  also  with  the  standards  of 
the  Jefferson  Physical  Laboratory. 

As  a  standard  electrode  the  mercury-calomel-potassium  chloride  electrode 
first  suggested  by  Ostwald"  was  considered  the  best  for  the  reasons  he  so 
convincingly  puts  forward  in  his  argument  with  Wilsmore."  In  the  present 
research  tenth  normal  instead  of  normal  potassium  chloride  was  usually 
used  as  the  electrolyte.  All  values  of  electromotive  force  are  given  for  the 
whole  cell,  Fe,  FeS04,  KCl,  HgCl,  Hg,  a  definite  measurable  quantity, 
instead  of  for  the  single  potential  difference  Fe,  FeSO^,  because  of  the  uncer- 
tainty as  to  the  absolute  value  of  the  decinormal  electrode.  If  0.56  volt  is 
supposed  to  be  the  value  of  the  normal  calomel  electrode,  the  value  0.612 
volt  for  the  tenth-normal  electrode  should  be  used,  as  recently  determined 
by  Sauer"  and  verified  by  us.  Subtracting  this  from  the  values  given,  a 
value  comparable  to  the  commonly  accepted  values  of  single  potential  differ- 
ences will  be  obtained. 

A  special  arrangement  of  the  decinormal  electrode  was  adopted,  in  order 
to  prevent  its  contamination  with  iron.  To  the  connecting-tube,  used  for 
immersion  in  the  ferrous  sulphate  cell,  was  attached  above  a  siphon-inlet 
from  a  large  bottle  of  decinormal  potassium  chloride  on  the  shelf  above. 
The  fact  that  it  was  possible  to  wash  out  the  tube  of  the  decinormal  elec- 
trode by  means  of  this  fresh  solution  removed  the  possibility  of  variation 
through  the  backward  diffusion  of  normal  ferrous  sulphate  or  other  electro- 


•Sackur,  Z.  f.  Electrochem.,  10,  843  (1904). 
"  Ostwald-Luther,  Physico-Chemische  Messungen,  p.  367. 
"Lehrbuch,  2,  945  (1893). 

^^Z.  f.  Physik.,  ch.  35,  333  (1900)  ;  ibid.,  36,  91  (1901). 

"Z.  f.  Physik.,  ch.  47,  184  (1904).    That  is,  N/10  electrode  =  0.612  volt  if  the  N/i 
electrode  is  called  0.560  volt. 


MEASUREMENT   OF   ELECTROMOTIVE   FORCE.  9 

lyte.  By  means  of  this  arrangement  one  such  electrode  lasted  through  a 
whole  season  and  at  the  end  of  that  time  gave  no  test  for  iron  in  the  cell 
itself.  Thus  the  uncertainty  of  this  possible  error  is  removed  and  the 
nuisance  of  repeated  renewal  of  the  electrode  done  away  with.  The  arrange- 
ment is  shown  on  the  left-hand  side  of  figure  i. 

The  usual  arrangement  of  the  iron  and  of  the  cell  was  as  shown  in  figure  I, 
and  on  a  larger  scale  in  figure  2.     The  sample,  about  half  the  size  of  a  pea, 


Fig.  I. — The  Cell  and  its  Appurtenances. 


A.  Reservoir  of  decinormal  potassic 
chloride. 

J3.  Reservoir  of  normal  ferrous  sul- 
phate containing  pure  iron 
wires. 

C.  Calomel  electrode   (decinormal). 


D.  Connecting      receptacle      containing 

ferrous  sulphate. 

E.  Inverted   U  tube. 

F.  Iron  electrode  (enlarged  in  fig.  2). 

G.  Automatic   hydrogen  generator,   pro- 

tecting Reservoir  B  from  air. 


was  inserted  in  a  running  noose  of  platinum  wire,  the  end  of  which  projected 
through  a  fine  hole  in  a  cork  stopper.  The  noose  was  then  pulled  tight, 
drawing  the  sample  against  the  cork,  and  the  wire  was  fastened  by  winding 
around  the  shank  of  an  iron  tack  on  the  other  side  of  the  cork.  The  cork 
was  fitted  into  one  end  of  a  glass  tube  and  carefully  sealed  with  molten 
paraffin.     The  tube  was  filled  with  normal   ferrous  sulphate  solution  by 


10 


ELECTROMOTIVE   FORCE  OF   IRON   AND   OCCLUDED   HYDROGEN. 


means  of  a  siphon  from  the  reservoir,  and  an  inverted  (J  "tube  and  stopper 
were  inserted  in  the  top  to  make  connection  with  an  open  test-tube  contain- 
ing ferrous  sulphate  fastened  alongside  by  means  of  a  rubber  band.  In  this 
way  the  iron  was  kept  for  some  time  safe  from  oxidation,  yet  always  open 
to  measurement.  The  arrangement  is  illustrated  in  figure  2.  When  the 
end  of  the  tube  of  the  decinormal  electrode  (C  in  figure  2)  was  dipped  into 
this  test-tube  full  of  ferrous  sulphate,  the  cell  was  complete  and  ready  for 
measurement.     A  large  number  of  "  iron  electrodes  "  could  be  placed  around 


Ky 


Q. 


I \ 


^ 


v-y  \^ 


Fig.  2.  Fig.  3. 

Receptacles  for  Iron  Electrodes. 


A 


o 


Fig.  4. 


C.  Tube    leading    to 

trode. 
F.  Contains     normal 

solution. 
O.  Protecting  layer  of  paraffin  oil. 


decinormal    elec- 
ferrous     sulphate 


P.  Hard  paraffin. 
V.  Iron   wire. 

W.  Platinum  wire  and  loop  for  holding 
porous  iron. 


the  sides  of  the  thermostat,  all  attached  to  one  wire,  and  the  change  of  con- 
nection from  one  cell  to  another  was  made  by  simply  lifting  the  iron  stand 
to  which  the  decinormal  electrode  was  attached  and  dipping  the  end  of  the 
standard  electrode's  tube  into  the  next  test-tube.  Thus  a  large  number  of 
measurements  could  be  taken  with  a  minimum  expenditure  of  time  and 
trouble,  a  very  necessary  matter  in  an  investigation  involving  thousands  of 
measurements.  The  thermostat,  electrically  regulated  and  heated,  in  which 
the  electrode  was  immersed,  was  kept  at  20.0°   C.     The  normal  ferrous 


THE  EFFECT  OF  PRESSURE. 


II 


sulphate  used  was  prepared  by  dissolving  almost  the  necessary  amount  of 
pure  crystals  in  a  quantity  of  water,  adding  a  small  measured  amount  of 
sulphuric  acid,  and  making  up  to  a  liter.  Then  pieces  of  piano  wire,  care- 
fully cleaned,  were  added.  The  acid  acting  on  the  iron  thoroughly  reduced 
the  solution,  the  amount  of  the  acid  having  been  so  chosen  that  just  the 
necessary  amount  of  iron  dissolved  to  make  the  solution  normal.  Several 
months  were  needed  to  remove  the  last  traces  of  acid — an  essential  condition. 
The  flask  was  connected  with  an  automatic  hydrogen  generator  whose  cock 
was  always  open,  and  when  liquid  was  withdrawn  through  the  siphon 
hydrogen  took  its  place.  This  also  is  represented  in  figure  i.  Thus  after 
having  once  prepared  the  solution,  it  could  be  kept  indefinitely,  perfectly 
constant  and  always  at  hand. 

THE  EFFECT  OF  PRESSURE. 

In  the  following  work  the  term  "  porous  iron  "  will  be  used  for  the  sake 
of  brevity  to  designate  the  iron  which  had  been  reduced  from  oxide  in  the 
manner  described  on  pages  6  and  7. 

The  results  of  a  series  of  preliminary  experiments  with  this  porous  iron 
are  given  in  table  i. 

The  first  two  were  made  with  samples  which  had  merely  suflfered  exposure 
to  air.  The  next  four  had  received  further  treatment,  having  been  put 
into  a  small  iron  mortar  such  as  is  used  in  powdering  rocks,  and  beaten  with 
a  heavy  hammer.  Under  this  treatment  the  friable,  porous  iron  became 
compact  and  gained  a  shining  metallic  surface.  The  electromotive  forces 
of  these  samples  were  then  measured  after  immersion  in  normal  ferrous 
sulphate.  The  cracked  and  powdered  edges  of  numbers  2  and  3  were 
covered  with  shellac,  so  that  only  the  smooth  metallic  surface  was  exposed 
to  the  solution. 

Table  i. — Preliminary  experiments — Electromotive  force  of  iron  against 
decinormal  electrode. 


Time 

elapsed 

after 

immersion. 

Sample  1, 

Sample  2, 

Sample  3, 

Sample  4, 

Sample  5, 

Sample  6, 

porous 

porous 

beaten 

beaten 

beaten 

beaten 

iron. 

iron. 

iron. 

iron. 

iron. 

iron. 

Minute. 

Volt. 

Volt. 

Volt. 

Volt. 

Volt. 

Volt. 

1 

0.704 

0.655 

0.655 

2 

0.674 



0.685 

4 

0.754 

0.755 



20 

0.780 

0.774 

0.724 

0.720 

0.674 

0.722 

55 

0.737 

0.728 

0.674 

0.736 

240 

6.801 

0.799 



12 


ELECTROMOTIVE   FORCE   OF   IRON   AND   OCCLUDED   HYDROGEN. 


A  glance  at  the  table  shows  that  the  beating  had  made  considerable  dif- 
ference in  the  electromotive  force  of  the  iron  in  every  sample  which  had 
been  beaten.  It  will  be  shown  later,  however,  that  this  difference  was  due 
not  to  the  compression,  but  to  another  effect  of  the  treatment. 

This  effect  was  soon  found  to  be  closely  connected  with  another  relation 
to  be  observed  in  the  above  figures;  namely,  the  fact  that  after  immersion 
the  electromotive  force  of  each  piece  of  iron  begins  at  a  low  point  and  rises, 
attaining  constancy  at  a  maximum  only  after  some  time.  In  order  to  study 
this  phenomenon  more  closely,  six  other  small  pieces  were  immersed  in  the 
solution  and  readings  of  the  electromotive  forces  taken  at  short  intervals. 
The  numerical  results  are  given  in  table  2. 

Because  it  was  evident  that  the  electromotive  force  continued  to  rise  slowly 
for  a  very  long  time,  four  of  these  pieces  were  carefully  sealed  up  with 
paraffin  immediately  after  immersion  in  order  to  prevent  oxidation.  This 
was  done  in  order  to  determine  the  electromotive  force  of  the  final  equi- 
librium. After  three  days  No.  9  and  No.  10  were  opened  and  measured; 
and  after  five  days  No.  11  and  No.  12. 


Table  2. 


The  increase  of  electromotive  force  with  time — Irori  electrode 
against  decinormal  electrode. 


Time  elapsed 

after 
immersion. 

Sample  7. 

Sample  8. 

Sample  9. 

Sample  10. 

Sample  11. 

Sample  12. 

Volt. 

Volt. 

Volt. 

Volt. 

Volt. 

Volt. 

Oh     2m 

0.590 

0.690 



0      3.75 

0.645 

0.745 



0      6 

0.685 

0.764 

0    18 

0.714 

0.774 

0    50 

0.749 

0.785 

73      0 



0.789 

0.793 

78      0 

0.791 

0.794 

119      0 

0.782 

0.784 

120      0 

0.784 

0.786 

This  was  later  repeated  with  other  samples,  which  gave  similar  results. 
It  should  be  noted  that  after  several  days  a  pale-green  precipitate  of  ferrous 
hydroxide  was  always  found  in  the  tube,  but  this  seemed  to  exercise  no 
considerable  effect  on  the  potential  until  seven  or  eight  days  had  passed  and 
the  amount  had  become  large.  Even  then  by  shaking  the  constant  value  of 
the  potential  was  usually  regained. 

Thus  it  was  obvious  that  spongy  iron  which  has  been  exposed  to  the  air 
always  gives  at  first  an  abnormally  low  electromotive  effect,  which  slowly 
rises.      The   only   reasonable   hypothesis   capable   of   explaining   the   phe- 


THE  EFFECT  OF  HIGH  PRESSURE.  1 3 

nomenon  seems  to  be  the  conclusion  that  the  exposure  to  the  air  results  in 
the  formation  of  an  exceedingly  thin  coating  of  oxide  or  at  least  of  adsorbed 
oxygen,  and  that  when  this  coating  is  removed,  by  solution  or  reduction, 
the  true  electromotive  force  is  obtained." 

Accordingly,  the  iron  to  be  used  in  the  succeeding  experiments  was  sealed 
in  hydrogen.  The  tubes  were  broken  open  just  before  the  samples  were 
to  be  used,  and  their  electromotive  forces  v*^ere  measured  as  soon  as  possible 
afterwards.  In  this  way  the  exposure  to  air  was  as  short  as  possible  and 
the  theory  was  confirmed,  for  they  were  found  to  attain  their  maximum 
values  very  promptly. 

With  these  samples  a  more  careful  study  of  the  effect  of  pressure  was 
begun.  A  number  of  preliminary  experiments  were  made  with  pressures 
produced  by  30,000  and  60,000  pounds.  These  pressures  were  quite  suffi- 
cient to  cause  cold-welding  of  the  particles  of  the  porous  iron,  giving  firm, 
hard  plates  with  bright  metallic  surfaces.  The  area  of  these  surfaces  was 
seldom  more  than  the  half  of  a  square  centimeter,  hence  the  pressures  were 
approximately  30,000  to  60,000  kilograms  per  square  centimeter.  The 
compression  was  done  between  two  smooth  plates  of  steel  in  a  large  testing- 
machine  in  the  Engineering  Department  of  Harvard  University.  The 
pressure  was  great  enough  to  press  the  soft  iron  into  the  steel  plates,  leaving 
distinct  indentations.  The  iron  was  handled  only  with  iron  pincers.  Every- 
thing with  which  it  came  in  contact  was  kept  scrupulously  clean.  The  edges 
of  the  compressed  pieces  being  cracked  and  split,  they  were  embedded  in 
soft  paraffin  before  immersion  in  the  ferrous  sulphate  solution,  leaving 
exposed  for  measurement  only  the  center,  which  had  certainly  borne  the  full 
effect  of  the  pressure. 

It  appeared  that  iron  thus  treated  gave  results  in  no  way  different  from 
the  spongy  pieces  before  compression,  except  that  the  compact  specimens 
were  slower  in  reaching  their  maxima.  When  reached,  these  maxima  were 
essentially  identical  with  those  of  the  porous  specimens. 

Nevertheless,  it  seemed  worth  while  to  carry  the  matter  further.  Through 
the  kindness  of  Lieutenant-Colonel  J.  G.  Butler  in  charge,  it  was  possible 
to  subject  other  portions  of  pure  spongy  iron  to  still  greater  pressure  on  the 
magnificent  testing  machine  at  the  United  States  Arsenal  at  Watertown, 
Massachusetts.  This  machine  is  capable  of  not  only  exerting  a  weight-effect 
of  1,000,000  pounds,  but  of  measuring  with  great  accuracy  the  magnitude 
of  the  effect  it'  exerts.     Two  special  blocks  were  made  between  which  to 


"This  experience,  and  the  conclusion  derived  from  it,  agrees  with  that  of  Mutti- 
mann  and  Fraunberger  (Sitzber.  Akad.  der  Wiss.  Miinchen,  34,  201)  (1904).  Their 
investigation  was  published  after  this  part  of  our  research  had  been  completed,  and 
was  wholly  independent. 


14 


ELECTROMOTIVE   FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 


compress  the  iron.  In  order  to  resist  such  enormous  pressures  they  were 
made  of  the  very  finest  tool  steel,  and  after  the  lathe-work  was  finished  they 
were  quenched  to  a  condition  of  maximum  hardness.  They  were  frustra 
of  cones,  the  diameters  of  the  bases  being  about  i6  cm.  and  those  of  the 
tops,  between  which  the  iron  was  compressed,  about  3  cm.  Their  heights 
were  about  8  cm.  The  pressures  exerted  were  500,000  pounds  and  700,000 
pounds.  Exactly  how  much  of  this  pressure  bore  on  the  iron  test-piece  it 
is  impossible  to  say,  for  the  soft  iron  was  pressed  so  deeply  into  the  hard 
steel  that  there  was  some  possibility  of  the  whole  top  surface  (7  sq.  cm.) 
receiving  part  of  the  pressure.  On  the  other  hand,  if,  as  is  probable,  the 
test-piece  alone  bore  the  stress,  the  maximum  pressure  was  at  least  350,000 
kilograms  per  sq.  cm.,  for  the  surface  of  the  soft  iron  was  never  i  sq.  cm. 

In  order  to  make  more  striking  any  differences  which  might  result  from 
these  pressures,  samples  13  and  14  had  each  been  broken  in  two,  and  one 
piece  of  each  was  kept  in  a  spongy  condition,  in  order  to  compare  with  the 
other  compressed  one.  Thus  it  was  certain  that  all  treatment  previous  to 
measurement"  had  been  precisely  the  same,  except  for  this  one  matter  of 
compression.     Other  pieces  of  porous  iron  gave  essentially  the  same  results. 

As  in  the  previous  experiments,  the  edges  of  the  compressed  iron  were 
protected  with  paraffin,  so  that  the  exposed  surface  should  be  that  alone 
which  had  been  compressed. 


Table  3. — The  effect  of  previous  compression  on  the  electromotive  force — 
Iron  electrode  against  decinormal  electrode. 


Time 
elapsed 
after 
immer- 
sion. 

Sample 

13. 
porous. 

Sample  la 

after 
subjection 

to 
350,000  kg. 
pressure. 

Sample 

14. 
porous. 

Sample  U 

after 
subjection 

360,000  kg. 
pressure. 

Sample  15 

after 
subjection 

to 
350,000  kg. 
pressure. 

Sample  16 

after 
subjection 

to 
350,000  kg. 
pressure. 

Oh   llm 

0    28 

6      0 

21       0 

Volt. 

0.775 

0.781 
0.780 

Volt. 

0.7.55 
0.781 
0.782 

Volt. 

*0.'786 
0.787 
0.787 

Volt. 
0.731 

0.770 
0.782 

Volt. 

0.744 
0.783 
0  788 

Volt. 
0.722 

'6 '.750 
0.751 

The  results  with  samples  13  to  15,  inclusive,  lead  to  the  conclusion  that  no 
permanent  change  in  free  energy  had  resulted  from  the  compression.  They 
are  exactly  similar  to  those  obtained  with  the  lower  pressures.  The  appar- 
ently contradictory  result  of  sample  16  will  be  shortly  explained. 

As  a  step  toward  this  explanation,  it  is  necessary  to  call  attention  once 


"Including  the  temperature  to  which  it  had  been  exposed,  the  importance  of  which 
will  shortly  be  pointed  out. 


THE   PERFECT   ELASTICITY   OF   PURE   IRON.  15 

more  to  the  fact  that  the  compressed  pieces  at  first  yielded  too  low  an 
electromotive  force,  although  the  value  afterwards  rose  to  equal  that  of  the 
uncompressed  pieces.  An  explanation  for  these  low  values  is  to  be  found 
in  the  idea  already  mentioned  on  page  13,  there  advanced  to  explain  the  lag 
of  porous  iron  which  had  been  kept  for  some  time  in  a  desiccator.  It  is 
that  an  exceedingly  thin  coating  of  oxide  is  formed  on  the  iron  by  exposure 
to  the  air  and  thus  the  true  electromotive  force  is  masked  until  this  coating 
is  dissolved.  Most  of  the  samples  of  porous  iron  had  been  kept  a  whole 
day  in  a  desiccator  after  reduction  before  immersion,  while  sample  14  in 
table  3  had  been  kept  only  three  hours.  The  difference  in  the  speed  of 
attaining  the  maximum  value  is  very  marked.  This  theory  not  only  seems 
to  account  for  the  effect  of  long  exposure  to  air,  but  also  offers  a  simple 
explanation  for  the  slightly  lower  initial  potential  of  the  compressed  samples. 
The  length  of  time  during  which  the  compressed  pieces  were  exposed  to 
the  air  before  measurement  was  exactly  the  same  as  that  of  the  parallel 
porous  ones,  but  the  former  were  inevitably  heated  somewhat  in  the  act  of 
compression,  and  thus  an  oxide  coating  might  well  have  been  formed  which 
would  equal  in  effect  one  of  long,  slow  formation  in  a  desiccator.  The 
theory  probably  also  offers  the  most  satisfactory  explanation  of  the  exag- 
gerated deficiency  exhibited  by  No.  16  in  table  3.  The  blocks  between 
which  No.  16  was  compressed  had  been  three  times  subjected  to  such  enor- 
mous pressures  that  they  were  beginning  to  fail,  and  this  fourth  and  last 
time  the  blocks  split  with  bright  sparks  and  loud  reports,  and  threw  small 
pieces  in  all  directions.  Under  such  treatment  the  blocks  undoubtedly 
became  warm,  and  it  is  quite  to  be  expected  that  this  last  sample  should 
possess  the  heaviest  and  firmest  coating  of  oxide. 

Thus  it  is  fairly  safe  to  conclude  that  even  such  enormous  pressures  as 
300,000  kilograms  per  square  centimeter  produce  no  permanent  change  of 
free  energy  in  iron. 

It  is  interesting  to  compare  this  constancy  with  the  behavior  of  other 
properties  of  metals  on  subjection  to  great  stress.  Since  this  work  was 
done.  Dr.  StuU  and  one  of  us  has  found,  in  the  work  already  referred  to, 
that  after  subjection  to  pressures  up  to  500  atmospheres  all  of  thirty-odd 
elements  examined  regain  precisely  the  original  volume.  The  method  used 
is  unique  and  allows  of  the  detection  of  volume  changes  of  0.000,01  milliliter 
in  a  sample  of  10  milliliters. 

After  this  work  was  done  we  became  acquainted  with  two  pertinent 
papers  by  W.  Spring,"  in  which  he  proves  the  same  fact  by  still  another 
method,  the  one,  in  fact,  which  would  most  naturally  be  adopted  if  that 
were  the  main  object  of  the  research,  namely,  the  determining  of  the  spe- 

"Bull.  Acad.  Roy.  de  Belg.  (3),  6,  509  (1883)  ;  Ibid.  (1903),  p.  1066. 


l6  ELECTROMOTIVE   FORCE   OF   IRON   AND   OCCLUDED   HYDROGEN. 

cific  gravity  before  and  after  compression.  He  first  submitted  to  pressures 
up  to  10,000  atmospheres  more  than  eighty  soHds  of  the  most  diverse  chem- 
ical nature,  and  found  that  powders  weld  together  perfectly  under  pressure, 
as  we  found  with  iron.  In  order  to  do  this,  he  found  it  necessary  to  com- 
press in  a  vacuum,  because  imprisoned  air  not  only  prevented  contact  of  the 
particles,  but  also  tore  them  apart  again  when  pressure  was  released.  All 
of  his  work  was  performed  with  the  care  which  this  precaution  indicates. 
The  result  of  one  investigation,  entitled  *'  L'elasticite  parfaite  des  corps 
chimiquement  definis,"  is  best  given  in  his  own  words: 

La  plupart  des  corps  examines  ont  manifeste  une  petite  augmentation  permanente 
de  densite  apres  avoir  subi  une  pression  d'environ  20,000  atmospheres,  mais,  celle-ci 
une  fois  realisee,  ils  ont  resiste  opionatrement  a  toute  diminution  permanente  ulterieure 
de  leur  volume.  Pour  tous  la  densite  a  atteint  bientot  un  maximum.  La  faible  aug- 
mentation de  densite  permanente  observee  n'a  cependant  jamais  eu  pour  cause  une 
contraction  reelle  de  la  matiere.  Tou jours  elle  a  ete  due  a  I'ecrasement,  par  pression, 
des  cavites  existant  d'avance  dans  le  corps  examine,  ou  bien  a  la  disparition  de  fissures 
plus  ou  moins  evidentes.  En  un  mot,  on  s'est  trouve  en  presence  d'un  fait  accidentel, 
pouvant  facilement  amener  une  erreur,  mais  non  d'un  phenomene  physique  essentiel. 

Thus  the  present  work  on  the  electromotive  force  of  iron  subjected  to 
great  stress  confirms  the  very  different  work  of  Spring  on  densities.  In  so 
far  as  these  data  show  that  when  no  change  of  specific  gravity  is  effected  by 
a  given  application  of  energy,  no  change  in  the  free  energy  of  the  system 
results,  they  support  the  theory  of  compressible  atoms. 

TENSION  EXPERIMENTS. 

Although  it  thus  appears  to  be  impossible  to  produce  permanent  free  energy 
changes  in  pure  iron  through  pressure  alone,  it  appeared  possible  that 
measurements  of  electromotive  force  made  while  the  metal  was  under  tensile 
stress  might  show  varying  values,  because  tension  produces  marked  physical 
changes  in  the  metal.  These  changes  are  as  follows:  (i)  A  strain  which 
immediately  disappears  on  release  of  the  tension,  inside  the  elastic  limit. 

(2)  Beyond  the  elastic  limit  a  permanent  strain,  with  increasing  stress 
needed  to  produce  equal  increment  of  strain,  up  to  the  ultimate  resistance. 

(3)  Beyond  the  ultimate  resistance,  with  ductile  metals,  there  is  a  region 
before  the  actual  break  occurs,  where,  for  increase  of  strain,  decreasing 
stress  is  required,  until  failure  takes  place.  It  seemed  reasonable  to  expect 
that  such  decided  and  sudden  changes  in  the  cohesion  would  be  accompanied 
by  free-energy  changes. 

While  under  stress,  or  after  a  "  permanent  set "  has  been  produced  by 
strain  beyond  the  elastic  limit,  the  iron  is  in  unstable  equilibrium.  Sufficient 
proof  that  it  is  so  within  the  elastic  limit  lies  in  the  fact  that  it  returns  of 


THE   WORK   OF   BARUS.  I'J 

itself  to  its  original  dimensions.  Beyond  the  elastic  limit,  the  custom  in 
wire  manufacture,  of  returning  the  wire  perhaps  half  a  dozen  times  to  the 
annealing  pots  to  be  softened,  speaks  for  itself.  By  heating  to  300°  to  400° 
for  several  hours  "  the  molecular  tension  in  the  wire  caused  by  drawing  " 
is  released."  The  molecules,  given  the  opportunity  through  increased  mo- 
bility at  higher  temperatures,  may  be  supposed  to  return  to  their  normal 
relations  to  each  other  and  plasticity  is  restored  to  the  iron. 

Further  evidence  in  this  direction  is  furnished  by  a  piece  of  work  done  by 
Barus,  entitled  "  The  Energy  Potentialized  in  Permanent  Changes  of  Molec- 
ular Configuration."  "  He  pulled  wires  of  various  metals  to  the  point  of 
failure,  determined  their  rise  in  temperature  by  means  of  a  thermopile,  and 
subtracting  the  heat  evolved  from  the  total  work  done  on  the  wires,  obtained 
values  for  the  amounts  of  energy  which  had  become  potential  in  the  metals. 
He  says: 

To  summarize,  it  appears  that  as  much  as  one-half  of  the  work  done  in  stretching 
up  to  the  limit  of  rupture  may  be  stored  up  permanently;  that  the  amount  of  work 
thermally  dissipated  varies  considerably  with  the  metal  acted  upon,  being  very  large, 
for  instance,  in  copper  (75  per  cent)  and  smaller  in  the  case  of  iron  (50  per  cent)  ; 
that  in  the  case  of  the  same  given  metal  the  work  is  largely  potentialized  during  in- 
cipient stages  of  strain,  and  very  largely  dissipated  during  final  stages  of  strain.  When 
stress  of  a  given  kind  is  applied  to  different  metals,  the  total  amount  of  energy  which 
can  be  stored  per  unit  section,  per  unit  of  length  up  to  the  limits  of  rupture,  may 
therefore  be  looked  upon  as  a  molecular  constant  of  the  metal. 

In  a  table  he  gives  results  showing  that  in  an  iron  wire  of  0.136  cm. 
diameter,  stretched  almost  to  the  limit  of  rupture,  at  least  2  megergs  per 
centimeter  are  potentialized,  about  the  same  amount  having  been  dissipated  as 
heat.  Knowing  this  value,  it  is  possible  to  calculate  the  maximum  rise  in  elec- 
tromotive force  theoretically  required  by  such  an  increase  in  the  free  energy 
of  the  metal,  supposing  that  all  this  work  were  available  as  free  energy.  The 
general  formula  for  such  a  calculation  is: 

Atz  =  

96s8o;;2 
m  which 

W  =  the  work  in  joules  done  per  centimeter  of  the  wire ; 

E  =  the  electrochemical  equivalent  of  the  metal,  in  grams; 

m  =  the  weight  of  the  metal  per  centimeter  of  wire ;  and 

W  X  E/m  =  the  work  done  per  gram  equivalent  of  the  metal. 

Substituting  values  found  by  Barus,  we  have: 

^7r  =  0.432  X  27.95/0. 1 1 14  X  1/96580  =  o.ooii  volt. 

(The  radius  of  the  wire  was  0.068  cm.  and  its  density  7.68.) 

"  "  Wire,  its  Manufacture  and  Uses,"  J.  B.  Smith,  pp.  30,  54,  and  56. 
"Amer.  Journ.  Sci.  (3),  38,  193  (1889);  also  U.  S.  Geological  Survey  Bulletin  No. 
94,  p.  loi  (1892). 


1 8  ELECTROMOTIVE  FORCE  OF   IRON   AND  OCCLUDED   HYDROGEN. 

Two  Other  calculations  will  be  given  to  illustrate  more  fully  the  changes 
to  be  expected.  The  data  for  these  were  found  in  a  book"  which  is  still 
quoted  as  an  authority  by  engineers,  although  published  as  long  ago  as  1879. 

In  the  first  of  these  (p.  24)  the  test-pieces  were  taken  "  from  a  bar  of 
remarkably  pure,  refined,  and  uniform  iron."  The  average  of  nine  tests, 
very  uniform  in  their  results,  gave :  Breaking  stress  per  square  inch,  55,489 
pounds ;  elongation,  23.9  per  cent.  The  diameters  of  the  test-pieces  were 
0.974  inch.  Calculating  as  before,  we  get  0.0034  volt  as  the  change  to  be 
expected. 

In  the  second  one  the  iron  is  described  as  "  very  soft  and  ductile."  The 
average  of  four  tests  gave :  Stress  per  square  inch,  45,873  pounds ;  elonga- 
tion, 29.7  per  cent.  The  diameters  were  1. 000  inch.  These  data  give  0.0035 
volt  as  the  expected  rise. 

The  results  of  all  these  calculations  should  be  divided  by  2,  since  Barus 
showed  that  only  50  per  cent  of  the  work  done  becomes  potential.  These 
calculated  values,  never  exceeding  1.8  millivolts,  represent  the  maximum 
values  for  the  increase  of  potential — values  which  may  not  be  actually 
attained,  because  it  is  by  no  means  certain  that  all  the  energy  thus  stored  is 
available  as  free  energy.  Moreover,  the  distribution  of  the  strain  between 
surface  and  interior  is  uncertain.  This  is  indicated  by  the  following  state- 
ment of  Burr :  "^ 

It  has  been  found  by  experiment  that  bars  of  wrought  iron  which  are  apparently 
precisely  alike  in  every  respect,  except  in  area  of  normal  section,  do  not  give  the 
same  ultimate  tensile  resistance  per  square  inch.  Other  things  being  the  same,  bars 
of  the  smallest  cross  section  give  the  greatest  intensity  of  ultimate  tensile  resistance. 

This  is  due  to  the  fact  that  shearing  strains  increase  with  greater  cross 
section.    And  also,"^ 

If  the  tensile  stress  is  uniformly  distributed  over  each  end  of  the  test-piece,  it  will 
not  be  so  distributed  over  any  other  normal  section.  Since  lateral  contraction  takes 
place,  the  exterior  molecules  of  the  piece  must  move  toward  the  center;  but  if  this 
motion  exists,  the  molecules  in  the  vicinity  of  the  center  must  be  drawn  farther  apart, 
or  suffer  greater  strains,  than  those  near  the  surface.  Hence  the  stress  will  no  longer 
be  uniformly  distributed,  but  the  greatest  intensity  will  exist  at  the  center  and  the  least 
at  the  surface  of  the  piece.  These  effects  will  evidently  increase,  with  a  given  form 
of  cross  section,  with  the  area. 

Smith,"  in  pointing  out  the  strengthening  effect  of  drawing  wire  has  said : 
"  In  the  case  of  the  wire  drawn  through  three  holes  the  tenacity  of  the 


" "  Experiments  on  the  strength  of  wrought  iron."  Report  of  the  Committee  of 
the  United  States  board  appointed  to  test  iron,  steel,  and  other  metals.  Commander 
L.  A.  Beardslee,  U.  S.  N.    Abridged  by  William  Kent.    New  York,  1879. 

*  Elasticity  and  resistance  of  the  materials  of  engineering,  p.  218.    New  York,  1903. 

"  Ibid.,  p.  206. 

""Wire,  its  manufacture  and  uses,"  p.  58. 


THE   SMALL   EFFECT   OF   TENSION.  I9 

metal  was  increased  95  per  cent  above  that  of  the  billet ; "  and  Burr  says :  ^ 
"  Wire  is  the  strongest  form  in  which  iron  can  be  used  to  resist  tensile 
stress." 

It  is  evident,  then,  that  by  pulling  a  wire  of  small  cross-sectional  area  it  is 
possible  to  exert  the  maximum  of  stress  on  the  surface  of  a  test-piece. 

The  apparatus  (shown  in  figure  4,  page  10)  employed  in  our  experiments 
was  exceedingly  simple.  The  upper  end  of  the  wire  was  twisted  around  an 
iron  support  and  the  pull  was  exerted  by  weights  in  a  basket  attached  to  the 
lower  end.  In  the  middle  the  wire  was  narrowed  at  one  place,  and  this  was 
surrounded  by  a  tube  of  normal  ferrous  sulphate  and  connected  with  a  nor- 
mal calomel  electrode.  The  results  were  at  first  highly  irregular.  The 
electromotive  force  of  the  wires  used  (pure  piano  wire  and  pure  soft  iron 
wire)  varied  over  a  range  of  0.005  volt  without  apparent  cause.  Merely 
tapping  the  stretched  wire  sometimes  caused  a  momentary  rise  of  from  0.003 
to  0.004  volt.  Such  electrical  disturbances  from  mechanical  jarring  have 
been  observed  by  von  Helmholtz.  In  one  case  the  addition  of  5  kilograms 
to  the  load  caused  a  net  rise  of  about  0.0007  volt  and  the  next  5  brought  it 
down  again  by  the  same  amount,  while  one  more  brought  it  up  again  by 
0.0005. 

Calculating  the  theoretical  change  of  potential,  we  get  0.0017  volt  for  the 
total  work  done,  and  half  of  this,  or  0.0009  volt,  as  the  change  to  be  expected 
according  to  Barus.  The  data  for  this  calculation  are:  Tensile  strength, 
25  kilograms ;  elongation,  9.3  per  cent ;  diameter  of  wire,  0.08  cm. 

Hence  the  accidental  disturbances  exceeded  the  maximum  theoretical  effect 
and  entirely  masked  the  latter. 

Only  one  regularity  was  noticeable,  namely,  that  at  the  instant  at  which 
the  load  was  increased  there  was  a  momentary  drop  in  electromotive  force. 
Starting  with  no  load  at  all  and  the  potentiometer  reading  0.7145  volt,**  a 
10  kg.  load  was  applied,  and  at  the  same  instant  the  galvanometer  swung 
18  to  20  divisions  to  the  right,  and  then  instantly  swung  back  again  very 
rapidly  to  the  permanent  reading,  0.7125  volt.  Exactly  how  much  fall  of 
potential  this  swing  to  the  right  indicates  could  be  determined  only  with  a 
ballistic  galvanometer,  but  it  may  have  been  as  much  as  0.015  volt.  The 
addition  of  another  kilogram  had  a  similar  effect,  except  that  the  swing  was 
only  12  to  15  divisions  to  the  right  and  it  returned  more  slowly  to  0.7125. 
The  third  10  kg.  caused  a  smaller  swing  still,  and  a  much  slower  return  to 
0.7125.     About  that  value  was  always  reached  after  standing  a  few  minutes. 

^  Elasticity  and  resistance  of  the  materials  of  engineering,  p.  241. 

'*This  reading,  being  taken  with  the  help  of  a  normal  calomel  electrode,  must  have 
0.052  volt  added  to  it  in  order  to  be  comparable  with  the  other  potentials  recorded  in 
the  paper.  It  thus  becomes  0.766,  the  same  as  before.  The  normal  electrode  has  less 
resistance  and  allows  more  sensitive  readings  than  the  decinormal. 


20  ELECTROMOTIVE   FORCE   OF   IRON   AND   OCCLUDED   HYDROGEN. 

The  weights  were  then  added  to  48  kg.,  the  last  ten  still  causing  a  slight 
swing  to  the  right.  A  13  kg.  weight  was  then  added  to  the  load,  and  the 
wire  lengthened  rapidly  and  would  have  undoubtedly  broken  had  not  the 
support  stopped  the  basket.  The  galvanometer  swung  8.5  divisions  to  the 
left,  indicating  a  rise  of  about  0.004  volt,  and  quickly  swung  back  again. 

These  temporary  changes  and  the  change  in  sign  which  they  undergo 
are  highly  interesting.  The  most  obvious  explanation  of  the  first  effect  of 
diminished  potential  is  to  ascribe  this  to  heating  incidental  to  pulling,  and 
experiment  gave  the  idea  some  support.  Heating  the  wire  to  redness  with 
a  small  gas  flame  just  above  the  surface  of  the  liquid  gave  a  somewhat 
variable  electromotive  force,  the  mean  value  being  about  0.015  volt  below 
the  initial  one — an  effect  about  like  the  first  effect  noted  above.  When 
cold  the  wire  returned  to  the  initial  value.  This  is  advanced  as  a  suggestion 
rather  than  a  definitive  explanation,  however. 

The  decreasing  temperature  effect  due  to  the  successive  additional  stresses 
may  have  been  due  to  the  larger  percentage  of  storing  as  potential  energy  as 
the  elastic  limit  is  approached — ^and  the  final  excess  of  potential  may  have 
been  due  to  the  sudden  manifestation  of  this  increased  potential  at  the  point 
of  fracture. 

Nevertheless,  these  conclusions  are  highly  hypothetical.  The  results  do 
not  permit  of  conclusions  broader  or  more  definite  than  these.  On  the  other 
hand,  they  afford  the  opportunity  of  correcting  two  pieces  of  work  which 
have  been  widely  published  and  whose  incorrect  results  have  been  given 
general  credence.  The  subject  is  of  such  great  importance  to  modern 
engineering  that,  naturally  enough,  engineers  were  the  ones  attracted  to  its 
investigation.  M.  P.  Wood,  in  his  book  "  Rustless  Coatings ;  Corrosion  and 
Electrolysis  of  Iron  and  Steel,"  ^  under  the  heading  ^'  Corrosion  Increased 
by  Stress  "  (p.  348),  quotes  three  men  as  entitled  to  a  hearing  on  this  subject. 

The  first  of  these,  Thomas  Andrews,^  carried  out  experiments  far  from 
satisfactory  from  the  electrochemical  point  of  view,  because  he  used  a  solu- 
tion of  common  salt  as  an  electrolyte,  and  prepared  the  two  pieces  of  iron 
for  comparison  in  different  ways.  The  observed  difference  in  potential  is 
quite  as  probably  to  be  ascribed  to  the  oxidation  effect  already  discussed  as 
to  the  effect  of  strain,  although  a  part  of  the  effect  may  have  been  due  to 
strain. 

The  second  paper  quoted  by  Wood  is  "  An  Experimental  Study  of  the 
Corrosion  of  Iron  under  Different  Conditions,"  by  Carl  Hambuechen."    Of 


"  Wiley  &  Sons,  N.  Y.,  1904. 
"Proc.  Inst  Civil.  Engin.  118,  356-374  (1894). 

'"Trans.  Amer.  Soc.  Mechan.  Engin.,  22,  816-821   (1901)  ;  or  Bull.  Univ.  Wisconsin, 
Engin.  Series,  vol.  2,  No.  8,  (1900). 


ERRORS   IN   PREVIOUS   INVESTIGATIONS. 


21 


this  paper  the  section  on  "  Corrosion  of  Strained  Metal,"  which  is  the  only 
part  to  be  discussed  here,  is  no  more  conclusive  than  Andrews's  work, 
although  Hambuechen  states  his  conclusions  in  unqualified  terms.  The 
work  was  very  carefully  performed  in  every  particular  except  one,  namely, 
the  fact  that  he  chose  ferric  chloride  solution  as  the  electrolyte  in  which  to 
measure  the  electromotive  force  of  the  iron  test-pieces.  This  unfortunate 
choice  completely  invalidates  all  the  results.  The  reason  is  very  simple. 
Ferric  chloride  is  always  hydrolyzed  to  a  considerable  extent — that  is,  it 
contains  acid.  When  such  a  solution  comes  in  contact  with  iron  the  three 
following  reactions  take  place : 

Fe  +  2Fe+++  =  3Fe++  (or  Fe  +  2FeCl3  =  sFeCl^), 

Fe  +  2H+  =  Fe++  +  H^  (or  2HCI  +  Fe  =  FeCl^  +  H^). 

H  (nascent)  +  Fe+++  =  Fe^"^  +  H+  (or  H  +  FeCla  =  FeCl^  +  HCl) . 

There  exists,  then,  at  the  electrode  a  continually  changing  concentration  of 
ferric,  ferrous,  and  hydrogen  ions,  and  reliable  work  is  out  of  the  question. 
The  effect  of  all  these  influences  combined  would  cause  the  electromotive 
force  to  change  steadily,  quite  independently  of  any  stress  and  strain  effects. 
The  rate  of  this  change  would  depend  on  such  indeterminate  and  chance  rela- 
tions as  the  area  of  the  iron  surface  to  the  volume  and  the  concentration  of 
the  solution,  and  on  the  mechanical  arrangement,  allowing  more  or  less 
rapid  diffusion,  etc.  The  following  experiments  are  enough  to  show  this. 
Wires  of  pure  iron  were  cleaned  with  fine  emery  cloth,  carefully  wiped,  and 
simply  immersed  in  a  ferric  chloride  solution.**  They  were  under  no  arti- 
ficial strain  of  any  kind  when  measured. 


Table  4. — The  electromotive  force  of  iron  in  ferric  chloride  against  the 
decinormal  electrode. 


Piano  wire  (20). 

Soft  wire  (31). 

Elapsed  time. 

E.  M.  F. 

Elapsed  time. 

E.  M.  F. 

Volt. 

Volt. 

Oh     0.^^ 

0.550 

Oh    im 

0.595 

0      5 

0.5725 

0      5 

0.608 

0    10 

0.576 

0    10 

0.610 

0    30 

0.585 

0    15 

0.610 

0    40 

0.588 

15    55 

0.718 

17      0 

0.743 

16    40 

0.716 

During  the  first  day  hydrogen  was  steadily  evolved  from  both  wires,  but 
the  second  day  this  side  reaction  was  greatly  diminished.  As  is  seen  in  the 
table,  the  electromotive  forces  rose  in  these  cases  respectively  0.192  and  0.123 


Similar  results  were  found  by  Finkelstein  in  1902.    (Z.  phys.  Cham.,  39,  91). 


^2  ELECTROMOTIVE   FORCE   OF   IRON   AND   OCCLUDED   HYDROGEN. 

volt.  This  rise  would  have  been  ascribed  by  Hambuechen  to  strain.  Further 
experiment  could  add  nothing  to  the  argument.  It  will  be  noticed  that  even 
finally  the  potential  was  very  far  from  the  value  in  ferrous  sulphate,  0.765. 

Hambuechen's  results  are  quite  as  irregular,  as  one  would  infer  from  these 
considerations.  For  example,  the  change  in  electromotive  force,  caused 
supposedly  by  tensile  stress,  varies  in  two  cases  from  0.004  to  0.056  volt,  and 
yet  in  the  former  case  the  load  per  square  inch  was  actually  600  pounds 
greater  than  in  the  latter.  Hambuechen  realized  that  some  of  his  observed 
potentials  were  much  larger  than  were  to  be  expected,  and  therefore  ex- 
plained that  "  One  of  the  assumptions  it  was  necessary  to  make  is  that  the 
stress  equally  affected  the  entire  cross  section,  which  is  by  no  means  the 
case,  for  it  is  the  outside  layer  which  is  affected  the  most  and  would  there- 
fore give  higher  values  of  electromotive  force  than  the  calculated  amounts." 
This  is  exactly  the  opposite  assumption  to  that  demanded  by  the  reasonable 
explanation  of  Burr  already  quoted  (p.  18). 

From  the  theoretical  point  of  view,  the  matter  appears,  then,  to  be  more 
complicated  than  might  be  inferred  from  a  hasty  survey  of  it.  One  can  not 
safely  conclude  that  the  potential  on  the  surface  of  a  wire  is  an  index  of  the 
free-energy  change  in  the  interior  of  the  wire ;  and  one  can  not  assume  that 
the  electrochemical  behavior  of  wire  while  under  stress  is  the  same  as  when 
the  stress  is  removed.  Still  further,  one  can  not  but  believe  that  the  perma- 
nent strain  caused  by  a  stress  (that  is  to  say,  the  energy  potentialized  by  a 
stress)  will  differ  in  the  case  of  impure  iron  from  that  in  the  case  of  pure 
iron.  Moreover,  it  is  by  no  means  certain  that  energy  stored  in  iron  by 
change  of  internal  structure  is  available  as  free  energy. 

Taking  all  these  considerations  together,  and  weighing  also  the  electro- 
chemical defects  in  some  of  the  papers  just  discussed,  it  is  apparent  that 
none  of  these  investigations  furnish  conclusive  evidence  concerning  the 
magnitude  of  the  changes  of  potential  to  be  noticed  on  stretching  a  rod  or 
wire.  It  is  apparent,  moreover,  that  this  change  is  smaller  than  is  usually 
supposed,  although  it  is  probable  that  a  slight  change  of  electromotive  force 
is  caused  by  a  tensile  strain  of  impure  iron. 

Qualitative  evidence  on  this  point  is  furnished  by  a  paper  entitled  '^  The 
Effect  of  Strain  on  the  Rate  of  Solution  of  Steel,"  published  by  Barus  in  the 
Bulletin  94  of  the  U.  S.  Geological  Survey  (p.  61),  already  quoted.  He 
experimented  with  soft  steel,  hard-drawn  steel,  and  the  latter  annealed;  he 
determined  the  rate  of  solution  by  loss  of  weight,  and  he  concluded  his 
report  as  follows :  "  Summarizing  the  above  results  as  a  whole,  it  follows 
that  the  rate  of  solution  of  drawn  steel  is  greater  than  the  rate  of  the  same 
homogeneous  metal  similarly  circumstanced."     In  applying  the  results  it 


VARYING  THE  TEMPERATURE  OF   IGNITION.  2$ 

should  be  noted  that  steel  alone,  not  pure  iron,  was  experimented  upon ;  that 
any  speed  of  reaction  in  which  two  phases  are  involved  is  complicated  by 
many  disturbing  factors ;  and  that  the  tendency  of  iron  to  occlude  hydrogen, 
discussed  in  a  later  section  of  this  paper,  must  have  added  another 
complication.'" 

Because  our  results  obtained  by  applying  both  compressing  and  distending 
stresses  upon  pure  iron  had  been  chiefly  negative,  indicating  very  slight 
changes  of  free  energy  for  large  stresses,  attention  was  now  directed  to  the 
change  of  the  other  conditions  of  experiment,  with  the  hope  of  tracing  to 
these  other  conditions  the  fairly  large  changes  of  electromotive  force  actually 
to  be  observed  in  specimens  of  pure  iron  prepared  in  different  ways. 

THE  EFFECT  OF  VARYING  THE  TEMPERATURE  OF  IGNITION. 

It  will  be  noticed  on  studying  tables  2  and  3  that  the  electromotive  forces 
of  cells  made  from  different  samples  of  iron  which  were  chemically  alike 
varied  between  such  wide  limits  as  0.780  to  0.794  volt,  a  range  far  beyond 
the  limits  of  experimental  error. 

In  tracing  these  differences  back  to  their  fundamental  cause,  the  first  clue 
was  furnished  by  the  different  degrees  of  cohesion  or  compactness  exhibited 
by  the  several  specimens.  When  the  four  pieces  of  porous  iron,  the  results 
from  which  are  given  in  table  3,  were  removed  from  the  cells  at  the  comple- 
tion of  the  electrochemical  experiments,  they  were  examined  with  regard  to 
their  cohesion  by  the  simple  method  of  resistance  to  pressure  and  to  fracture 
between  the  fingers.  It  was  thus  found  that  No.  2  iron  felt  softest.  No.  3 
the  hardest  and  firmest,  and  Nos.  i  and  4  somewhere  between.  Reference 
to  the  table  shows  that  No.  2  had  the  highest  electromotive  force  and  No.  3 
the  lowest;  hence  a  more  powdery  structure  evidently  went  with  greater 
tendency  to  dissolve.     Repetition  with  other  samples  confirmed  this  view. 

What,  now,  could  be  the  cause  of  this  difference  in  compactness  of  the  dif- 
ferent samples?  It  is  well  known  that  many  substances,  on  being  heated, 
begin  to  cohere  at  temperatures  far  below  their  melting-points,  exhibiting 
the  shrinkage  commonly  called  "  sintering."  It  is  also  well  known  that  this 
effect  becomes  more  and  more  noticeable  as  the  melting-point  is  approached. 
Probably,  then,  these  different  samples  had  been  subjected  to  different  tem- 
peratures during  their  reduction.  This  relationship  was  easily  confirmed 
by  further  experiment.  Iron  reduced  at  high  temperatures  was  found  to  be 
far  firmer  in  structure  than  that  reduced  at  low  temperatures.  In  order  thus 
to  find  the  highest  potential  attainable  in  this  way,  iron  should  obviously  be 


*•  In  this  connection  attention  is  called  to  the  interesting  work  of  C.  S.  Burgess  on 
the  effect  of  impurities  on  the  rate  of  solution,  read  at  the  recent  meeting  of  the 
American  Electrochemical  Society,  May,  1906. 


24 


ELECTROMOTIVE   FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 


reduced  at  the  lowest  possible  temperature.  Accordingly  two  samples  were 
prepared,  the  first  being  very  slowly  reduced  at  a  temperature  well  below 
800°.  In  the  second  of  these  reductions  the  temperature  was  measured  by  a 
Le  Chatelier  thermopile  laid  under  the  boat  containing  the  iron,  and  the 
highest  temperature  it  indicated  through  a  reduction  lasting  three  days  was 
615°.  The  iron  in  boat  No.  i  was  of  a  clear  gray  color  in  those  parts  where 
the  temperature  became  highest  and  dark  blackish  gray  on  the  surface.  All 
the  iron  in  boat  No.  2  was  like  the  top  of  boat  No.  i.  Even  the  firmest  iron 
in  boat  No.  i  could  easily  be  crushed  between  the  fingers. 

Two  samples  of  the  light-gray  material  from  boat  No.  i  gave  the  follow- 
ing measurements : 


Table  5. — The  electromotive  force  of  porous  iron  reduced  at  low  temperature. 


Sample  22. 

Sample  23. 

Elapsed  time. 

E.  M.  F. 

Elapsed  time. 

E.  M.  F. 

Oh       ^m 

0  30 

1  0 
24       0 
70     10 

Volt. 
0.796 
0.800 
0.798 
0.799 
0.796 

Oh  14m 

1       0 

24       5 

70     40 

92       0 

Volt. 
0.797 
0.797 
0.799 
0.800 
0.800 

These  values  are  somewhat  higher  (0.002  and  0.006  volt)  than  any  ob- 
tained in  previous  work,  and  are  further  evidence  in  favor  of  believing  that 
the  low  temperature  of  reduction  really  causes  a  product  having  a  higher 
potential.  The  contents  of  boat  No.  2  were  found  to  contain  much  hydrogen, 
and  will  be  discussed  later. 

Having  thus  shown  that  iron  reduced  at  low  temperature  may  give  in 
combination  with  the  decinormal  electrode  a  potential  as  high  as  0.800  volt, 
and  that  increasing  temperature  of  reduction  causes  a  steady  decrease  in  this 
value  until  the  metal  prepared  at  perhaps  1,200°  gives  a  value  as  low  as  0.776, 
it  became  a  matter  of  interest  to  discover  if  iron  subjected  to  a  still  higher 
temperature  might  not  give  a  still  lower  value.  The  obvious  subsequent 
step  was  therefore  to  test  iron  which  had  been  fused,  and  accordingly  several 
specimens  of  the  purest  commercial  iron  (Swedish  bar  iron  and  pure  piano 
wire)  were  tested  in  preliminary  experiments.  The  Swedish  bar  iron  was 
very  pure,  and  soon  reached  a  constant  value.  The  piano  wire  was  less 
regular,  and  hence  was  examined  more  thoroughly.  The  wire  was  "  hard 
drawn,"  and  its  purity  was  indicated  by  the  fact  that  when  heated  to  1,000° 
in  hydrogen  and  quenched  in  ice-water  it  was  as  soft  and  pliable  as  the 
purest  iron.     Two  pieces  of  this  wire  were  scraped  bright  and  clean  with  a 


COMPACT   IRON   PREPARED   AT   HIGH   TEMPERATURE. 


25 


dull  knife  and  measured  as  usual  in  a  cell  shown  in  figure  3,  page  10.  The 
potential  of  these  cells  began  at  very  low  value,  and  gradually  rose  to  a  con- 
stancy at  about  0.76  volt.  It  was  evident  that  the  end-point  is  reached  only 
after  a  long  time;  and  accordingly  six  wires  were  sealed  up  in  ferrous 
sulphate  solution.  Three  of  these  had  been  scraped  as  before  to  clean  them 
and  the  other  three  were  rubbed  with  fine  emery  cloth,  all  of  them  being 
carefully  wiped  with  a  cloth  before  immersion.  Below  are  given  the  final 
constant  values  attained  by  these  samples : 

Table  6. — The  electromotive  force  of  commercial  iron  of  moderate  purity. 


Elapsed 
time. 

Swedish 
bar  iron. 

Piano  wire  (hard  drawn). 

Sandpapered. 

Scraped. 

No.  24. 

No.  25. 

No.  26. 

No.  27. 

No.  28. 

No.  29. 

No.  30. 

3  hours 

1  day 

3  days 

4  days 

7  days 

Volt. 
0.765 
0.770 
0.770 

Volt. 

0.762 
0.762 
0.762 

Volt. 

0.762 
0.762 
0.763 

Volt. 

'  'o*.762 
0.762 
0.763 

Volt. 

"  0.767 
0.763 
0.765 

Volt. 

'o!769 
0.767 
0.767 

Volt. 

0.765 
0.765 
0.763 

Thus  all  these  samples  yield  a  result  lower  than  the  most  compact  of  the 
sintered  samples.  The  results  show  excellent  agreement,  and  the  number 
0.765  volt  may  be  taken  as  the  true  electromotive  force  of  the  cell  made 
from  this  wire.  The  Swedish  iron  gave  a  slightly  higher  value,  about  0.770. 
A  third  sample  of  commercial  iron,  the  softest  and  -purest  wire  obtainable, 
was  next  examined.  It  was  cleaned  with  fine  emery  cloth  and  wiped  with  a 
clean  cotton  cloth.  Two  specimens  after  two  days  gave  respectively  0.760 
and  0.771  volt,  or  in  mean  0.766,  essentially  the  same  as  before. 

Thus  all  the  commercial  compact  samples  of  iron  showed  a  distinctly  lower 
solution  tension  than  any  of  the  spongy  pure  samples.  Before  this  differ- 
ence could  be  certainly  ascribed  to  any  peculiarity  in  the  freshly  reduced 
spongy  iron,  proof  must  be  afforded  that  it  was  not  due  to  impurities  in  the 
commercial  samples.  This  possibility  was  most  conveniently  subjected  to 
trial  by  fusing  a  sample  of  pure  reduced  iron,  and  comparing  the  product 
with  the  commercial  sample.  At  first  an  attempt  was  made  to  prepare  pure 
fused  iron  by  fusing  a  small  quantity  in  a  small  lime. crucible  in  an  oxyhy- 
drogen  flame.  The  hydrogen  used  was  very  pure,  having  been  made  by 
electrolysis,  for  impurities  would  be  taken  from  the  gas  by  the  fused  iron. 
The  flame  was  so  directed  as  to  cover  the  iron  completely  and  was  provided 
with  excess  of  hydrogen,  but  nevertheless  all  the  iron  oxidized.  The  oxide 
unfortunately  did  not  act  as  a  protection  to  the  iron  underneath,  because  it 
fused  and  was  absorbed  by  the  porous  lime.     No  acceptable  variations  of 


26 


ELECTROMOTIVE   FORCE  OF   IRON   AND  OCCLUDED  HYDROGEN. 


apparatus  or  manipulation  greatly  bettered  the  result,  which  could  have  been 
easily  accomplished  with  larger  quantities  of  material.  Therefore  another 
method  was  tried,  namely,  the  melting  of  the  iron  in  a  vacuum  by  means  of 
the  heat  generated  by  its  own  resistance  to  a  great  electric  current.  Be- 
sides an  apparatus  suitable  for  exhaustion,  the  essentials  for  success  were 
iron  electrodes,  a  suitable  crucible,  and  a  sufficiently  high  current.  Bundles 
of  piano  wire  provided  the  electrodes,  and  a  nest  of  powdered  iron,  sup- 
ported on  a  crucible  of  lime  made  from  the  purest  marble,  was  used  to 
contain  the  melted  particles.  In  order  to  increase  the  resistance  of  the 
metal  it  was  powdered  in  the  Hempel  steel  mortar,  and  the  current  was 
passed  through  this  powder.  The  arrangement  of  the  apparatus  and  its 
description  are  given  in  figure  5. 

The  operation  of  the  experiment  was  as  follows:  After  everything  was 
arranged  as  shown  in  the  figure,  the  Sprengel  vacuum-pump  was  started 
and  the  stoppers  thus  firmly  forced  in.  This,  however,  disarranged  the  elec- 
trodes and  air  was  again  admitted,  so  that  they  could  be  carefully  placed  at 
the  desired  places  in  the  lime  boat.  The  apparatus  was  now  evacuated  a 
second  time  and  the  current  turned  on.  For  some  time  nothing  happened, 
but  finally  the  metal  fused  with  the  help  of  vigorous  jarring.  More  and 
more  powder  was  added  from  above  as  the  fusion  progressed.  Tliere  were 
brilliant  flashes  of  violet  light  with  each  addition  of  powder,  followed  by 
bright-red  glowing.  By  feeding  powder  into  the  open  spaces  left  by  the 
fused  material  it  was  possible  to  maintain  the  fusion  almost  continually  until 
the  supply  was  exhausted.  The  product  consisted  mainly  of  small  pellets 
about  the  size  of  the  head  of  a  pin  or  a  little  larger,  resting  in  the  nests  of 
pure  powdered  iron.  Only  one  was  large  enough  to  use  conveniently  in  a 
cell.  Nevertheless,  this  gave  a  result  agreeing  so  well  with  that  from  the 
commercial  materials  that  a  duplicate  seemed  unnecessary.  The  sample  was 
covered  with  soft  paraffin  on  those  parts  which  did  not  have  a  smooth  fused 
surface. 


Table  7. — Pure  iron  fused  in  vacuum.     (33). 


Elapsed  time. 

E.  M.  P. 

Elapsed  time. 

Oh       7m 
0         9 
0       20 
0       40 

Volt. 
0.745 
0.750 
0.757 
0.760 

40h       40m 

41       30 
72       10 
90       25 

E.  M.  F. 


Volt. 
0.761 
0.759 
0.763 
0.761 


Thus  this  sample  of  exceedingly  pure  iron  fused  in  vacuum  gave  essen- 
tially the  same  potential  as  good  quality  commercial  material  (0.765). 


PURE   IRON    HEATED   IN   VACUUM. 


n 


This  iron  must  have  been  wholly  free 
from  carbon,  from  any  other  metal,  and 
indeed  even  from  all  but  the  merest 
trace  of  hydrogen.  It  is  hence  clear  that 
the  impurities  in  the  commercial  iron 
could  not  have  been  enough  to  affect  its 
electromotive  force,  and  that  pure,  com- 
pact iron  really  gives  a  lower  value  than 
the  same  iron  reduced  by  hydrogen.  To 
what  cause,  then,  is  this  difference  to  be 
ascribed?  Is  the  higher  solution  tension 
of  the  spongy  iron  due  to  the  traces  of 
hydrogen  within  it,  or  to  difference  of 
internal  structure,  or  to  the  disposal  of  its 
external  surface? 

The  first  of  these  three  possible  causes 
of  an  extraordinarily  high  potential  does 
not  seem  to  be  a  very  probable  one; 
nevertheless,  it  seemed  worthy  of  test, 
especially  in  view  of  the  results  on  oc- 
cluded hydrogen  to  be  recorded  later. 
Accordingly,  several  samples  of  spongy 
iron  were  ignited  for  varying  lengths  of 
time  at  varying  temperatures  in  a 
vacuum,  while  inclosed  in  a  stout  porce- 
lain tube.  It  was  thus  found  that  while 
short  ignition  or  ignition  at  a  low  tem- 
perature left  much  of  the  gas  in  the 
metal,  treatment  at  a  higher  temperature 
for  a  longer  time  seemed  to  remove 
nearly  all  of  it,  while  causing  the  iron  to 
sinter  into  a  compact  mass.  Iron  thus 
treated  at  i,i6o°  gave  in  conjunction 
with  the  decinormal  electrode  the  usual 
potential  0.787  in  a  few  hours,  a  value 
which  remained  unchanged 
even  in  the  last  decimal  place 
after  ninety  hours.  Even  this 
iron,  however,  undoubtedly 
contained  traces  of  hydrogen,  although 
much  less  than  at  first. 


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26  ELECTROMOTIVE   FORCE  OF   IRON   AND   OCCLUDED   HYDROGEN. 

The  permanence  in  the  value  in  the  potential  is  in  itself  the  best  evidence 
that  the  hydrogen,  even  if  present,  is  so  shielded  from  the  electrolyte  as  not 
to  affect  the  measurement  of  electromotive  force.  It  is  improbable  that  a 
trace  of  occluded  gas  could  exercise  a  lasting  effect  in  maintaining  the 
potential  above  that  of  the  reversible  electrode  of  iron  in  its  sulphate.  This 
matter  will  be  discussed  in  detail  in  the  second  section  of  the  paper. 

There  are  two  other  hypotheses  capable  of  explaining  the  abnormal  poten- 
tial of  the  spongy  iron.  It  is  certain  that  the  "  sintering '  is  increased  and 
the  surface  of  the  iron  diminished  in  this  way ;  and  it  is  possible  that  change 
in  the  degree  of  polymerization  of  the  iron,  resulting  in  the  formation  of  a 
more  stable  modification,  is  likewise  effected.^"  Either  of  these  changes 
might  affect  the  electromotive  force.  A  method  of  deciding  between  these 
two  would  demand  either  the  formation  of  compact  iron  at  low  temperature, 
or  else  of  loose  spongy  iron  at  high  temperature — and  neither  is  capable  of 
certain  preparation.  The  former  was  nearly  obtained  in  the  experiment  first 
recounted,  in  which  the  spongy  iron  was  rendered  compact  by  great  pressure ; 
the  latter  can  hardly  be  obtained  by  mechanical  subdivision. 

It  is  to  be  noticed  that  of  the  first  three  specimens  used  in  the  experiments 
recorded  in  table  3,  the  untouched  porous  pieces  gave  an  average  potential 
of  0.783,  while  the  same  pieces  after  cold-welding  at  very  high  pressure  gave 
the  essentially  identical  value  0.782.  The  cold-welded  pieces  were  very  hard 
and  tenacious,  and  appeared  qualitatively  in  every  way  similar  to  iron  which 
had  been  fused  (which  gave  only  0.765  volt)  ;  but  it  is  not  certain  that  some 
particles  on  the  surface  were  not  as  loosely  coherent  as  in  the  sponge,  and 
hence  capable  of  giving  as  high  a  potential.  It  is  true  that  one  fact  mili- 
tates against  this  possibility — the  fact,  namely,  that  the  compressed  pieces 
steadily  rose  to  the  maximum  potential  and  staid  there  a  long  time,  which 
could  have  hardly  been  the  case  if  the  high  potential  had  depended  upon  a 
few  particles. 

Because  of  the  experimental  difficulties  it  is  impossible  now  to  decide  defi- 
nitely between  these  possible  causes  of  difference  between  the  spongy  and 
compact  metal.    It  is  possible  that  all  three  are  superposed ;  but  of  the  three. 


""The  allotropic  theory  of  iron  need  not  be  discussed  here.  It  has  satisfactorily 
explained  so  many  phenomena  and  has  had  such  wide  usefulness  that  it  is  universally 
accepted  by  scientific  metallurgists.  Its  theoretical  side  from  the  standpoint  of  the 
phase  rule  is  thoroughly  treated  by  Roozeboom,  Z.  f.  Phys.,  ch.,  34,  436  (1900).  The 
metallurgist's  use  of  it  is  shown  by  Baron  von  Juptner  in  his  work  "  Grundziige  der 
Siderologie,"  vol.  2,  1901.    See  also  Berichte  der  d.  Chem.  Gesell,  39,  2381  (1906). 

A  short  explanation  of  the  terminology  may  not  be  out  of  place.  Pure  iron  has  at 
least  two  well-defined  transition  points.  Iron  stable  below  766°  is  known  as  alpha 
iron,  between  766°  and  895°  as  beta  iron;  and  the  phase  formed  above  895°  is  called 
gamma  iron. 


THE   SUDDEN    COOLING   OF   PURE   IRON.  29 

the  writers  are  rather  inclined  to  believe  that  the  chief  cause  is  probably  the 
state  of  subdivision — the  very  finely  divided  particles  giving  a  higher  electro- 
motive force  than  the  fused  mass."  If  this  is  true,  the  case  is  one  of  great 
interest  because  of  the  magnitude  of  the  effect,  corresponding  as  it  does  to 
a  tenfold  greater  solubility  of  the  finest  particles. 

In  this  connection  it  was  deemed  worth  while  to  test  the  electromotive 
force  of  iron  which  had  been  suddenly  cooled  or  quenched.  In  this  way  it 
might  be  found  whether  or  not  pure  iron,  when  suddenly  cooled,  could  be 
caught  in  a  condition  of  internal  structure  less  stable  than  when  cooled 
slowly. 

Preliminary  experiments,  to  be  recounted  later  in  another  connection, 
showed  that  in  order  to  obtain  significant  results  for  this  purpose  the  heat- 
ing and  sudden  cooling  must  occur  in  the  absence  of  any  foreign  substance 
capable  of  reacting  upon  the  iron ;  therefore  the  metal  must  be  quenched  in 
the  absence  of  any  gas  or  water. 

The  apparatus  needed  was  not  complex.  A  stout  tube  of  Berlin  porcelain 
was  clamped  in  a  vertical  position,  closed  at  both  ends  by  Hempel  water- 
cooled  stoppers,  and  was  heated  in  the  middle  by  an  encompassing  perforated 
Fletcher  furnace.  The  temperature  was  registered  by  a  platinum-rhodium 
thermopile.  The  porcelain  plate  on  which  the  test-pieces  of  iron  were  laid 
was  suspended  in  the  middle  of  the  tube  by  an  iron  wire  on  one  side  and  a 
small  silver  wire  on  the  other.  On  the  lower  Hempel  stopper  lay  a  smooth, 
thick  iron  plate,  and  ice-cold  water  was  circulated  through  the  stopper, 
while  the  whole  lower  end  of  the  tube  was  packed  in  ice.  The  apparatus 
was  now  evacuated  with  a  Sprengel  mercury  pump,  and  when  the  pressure 
had  fallen  to  less  than  i  mm.  the  tube  was  heated.  For  ten  minutes  the 
temperature  was  kept  above  i,ooo°  ;  the  silver  wire  was  melted  and  one  side 
of  the  porcelain  plate  was  dropped.  The  iron  of  course  fell  from  this  region 
of  high  temperature  to  the  cold  iron  plate.  The  apparatus  was  left  to  cool 
over  night,  without  the  admission  of  a  trace  of  air.  The  two  pieces  of  iron 
when  put  in  had  been  blackish-gray  and  very  loosely  knit  together;  when 
taken  out  they  were  quite  cohesive  and  of  a  clear-gray  color,  exactly 
resembling  iron  which  had  been  reduced  in  hydrogen  above  700°.  This 
illustrates  the  effect  of  sintering  and  proves  that  the. original  dark  color  of 
the  iron  powder  was  not  due  to  incomplete  reduction,  but  solely  to  its  state 
of  division.  One  surface  of  one  of  the  two  pieces  was  found  lying  flat  on  the 
cold  iron  plate  and  this  one  was  immersed  in  ferrous  sulphate  and  measured. 
It  gave  in  eight  minutes  a  potential  of  0.788,  which  slowly  rose  to  0.805  in 
an  hour,  and  then  settled  down  to  constancy  at  the  value  of  0.795. 


Ostwald,  Zeitschr.  fur  Phys.  Chem.,  34,  45  (1900)  ;  Hulett,  ibid.,  37,  385  (1901). 


30  ELECTROMOTIVE   FORCE  OF   IRON   AND  OCCLUDED   HYDROGEN. 

In  repeating  the  experiment  two  improvements  were  introduced.  For 
fear  that  some  silver  might  have  volatilized  and  contaminated  the  iron,  the 
silver-wire  support  was  replaced  by  a  simple  mechanical  device,  one  side  of 
the  porcelain  plate  supporting  the  iron  being  held  in  place  by  a  pipe-stem, 
while  the  other  side  was  attached  to  an  iron  wire  running  through  a  lubri- 
cated hole  in  the  stopper.  By  pulling  the  wire  the  iron  could  be  dropped. 
Furthermore,  quick  and  effectual  cooling  was  obtained  by  surrounding  the 
outside  of  the  base  of  the  porcelain  tube,  containing  the  iron  cooling-plate, 
by  a  mixture  of  chloroform  and  solid  carbon  dioxide.  Time  was  allowed  for 
the  thorough  cooling  of  the  inclosed  iron  plate.  The  temperature  of  porous 
iron  was  thus  almost  instantaneously  lowered  from  i,ioo°  to  — 75°  or  there- 
abouts. Great  pains  was  taken  to  exclude  the  vapor  of  water,  which  would 
have  condensed  in  the  cold  part  of  the  tube  and  vitiated  the  experiment. 
This  was  accomplished  by  running  a  stream  of  pure  dry  hydrogen  through 
the  tube  for  many  hours. 

As  soon  as  possible  after  the  fall  of  the  porous  iron,  which  took  place 
this  time  in  an  atmosphere  of  hydrogen,  the  pieces  were  taken  out,  immersed 
in  ferrous  sulphate,  and  measured  in  the  usual  way.  In  six  minutes  after 
immersion  the  potential  was  0.790,  in  twenty  minutes  0.796,  and  in  an  hour 
0.798.  After  a  day  it  settled  down  to  perfect  constancy  at  0.793  volt,  essen- 
tially identical  with  the  result  of  the  previous  experiment.  This  is  not 
enough  above  the  usual  value  (0.787)  obtained  from  iron  slowly  cooled  from 
the  same  high  temperature  to  allow  the  conclusion  that  the  sudden  cooling 
had  made  any  considerable  difference  in  the  potential  or  difference  in  internal 
structure. 

It  becomes  now  an  interesting  question  as  to  which  of  the  many  values 
for  the  gradually  changing  potential  of  the  cell  Fe,  ftFeSO^,  V^oKCl,  HgCl, 
Hg  correspond  to  the  true  values  of  the  solution  tension  of  the  different 
forms  of  iron.  Consideration  shows  that  the  final  high  values  are  probably 
to  be  taken  in  each  case,  for  several  reasons.  In  the  first  place,  these  values 
are  approached  asymtotically,  and  are  then  maintained  at  a  constancy  for  a 
long  time.  For  example,  one  sample  of  iron  which  in  16  hours  had  given 
in  the  cell  a  potential  of  0.793  volt,  after  150  hours  more  was  still  practically 
unchanged  at  0.792  volt.  In  the  next  place,  the  different  kinds  of  iron, 
although  sometimes  beginning  at  impossibly  low  values,  all  finally  rise  to 
nearly  the  same  point.  Again,  it  is  not  conceivable  that  the  iron  should 
gradually  raise  itself  to  a  potential  above  its  true  value  and  maintain  itself 
there  in  a  reversible  reaction  with  a  large  excess  of  both  iron  and  ferrous 
sulphate  at  hand,  while  it  is  easy  to  see  that  a  coating  of  oxygen  would  lower 
the  potential  at  first.  The  conclusion  is,  moreover,  reinforced  by  the  fact 
that  the  same  potential  is  attained  from  the  opposite  direction  by  the  gradual 


THE  SINGLE  POTENTIAL  OF  THE   PURE   IRON   ELECTRODE.  3 1 

falling  off  of  a  higher  potential  artificially  and  temporarily  created  in  the 
metallic  electrode  with  the  help  of  hydrogen  conveyed  to  the  system  from 
without,  as  will  be  explained  in  the  succeeding  section  of  the  paper.  Hence 
there  can  be  little  doubt  that  the  potential  of  porous  iron  reduced  at  about 
1,000°  and  immersed  in  normal  ferrous  sulphate  is  really  as  high  as 
0.79 — 0.612  :=  0.18  volt,  if  the  decinormal  calomel  electrode  is  taken  as  0.612, 
the  normal  calomel  electrode  being  taken  as  0.56.  Pure  iron  which  has  been 
fused  in  a  vacuum  gives  a  value  nearly  0.03  volt  lower,  or  about  0.15  volt. 
This  latter  value,  corresponding  to  the  flat  surface  of  a  coherent  mass,  is  the 
one  to  be  chosen  ordinarily  as  the  normal  value.  Even  this  is  nearly  twice 
as  large  as  that  usually  ascribed  to  it.  The  errors  of  the  work  of  others  were 
probably  insufficient  time  of  immersion  and  the  presence  of  acid  in  the  electro- 
lyte, both  of  these  tending  to  lower  the  observed  value,  as  is  shown  above. 


PART  II.    THE   ELECTROMOTIVE   FORCE   OF   IRON   CON- 
TAINING OCCLUDED  HYDROGEN. 


The  fact  that  hydrogen  freely  penetrates  iron  at  red  heat  and  that  iron 
retains  some  of  this  hydrogen  on  cooHng  was  discovered  as  long  ago  as  1866 
by  Graham ;  but  the  more  detailed  study  of  this  and  related  phenomena  has 
been  undertaken  only  in  recent  years,  after  its  importance  in  the  metallurgy 
of  the  metal  had  been  established.  Many  papers  have  been  published  on  this 
topic,  but  the  field  has  been  by  no  means  thoroughly  treated. 

Our  attention  during  this  research  was  first  directed  to  the  well-known 
affinity  of  iron  for  hydrogen  by  observing  the  copious  evolution  of  this  gas 
from  metal  reduced  at  a  low  temperature.^  Iron  obtained  from  the  oxide  at 
570°  is,  as  well  known,  a  fine  powder,  which  oxidizes  easily,  and  hence  after 
even  a  short  exposure  to  the  air  gives  a  low  electromotive  force.  In  order 
to  test  properly  the  free  energy  of  such  iron,  it  must  be  cooled  in  pure  hydro- 
gen and  tested  instantly  after  opening  the  reduction-tube.  In  this  way  it 
was  found  that  the  potential  is  not  essentially  different  from  iron  reduced 
at  700°,  being  about  0.795  volt  in  conjunction  with  the  decinormal  electrode. 

Nevertheless,  such  powder,  upon  immersion  in  ferrous  sulphate  evolved 
copious  bubbles  of  gas,  which  were  proved  to  be  hydrogen.  That  this  hydro- 
gen was  not  in  any  extraordinary  condition  is  shown  by  the  fact  that  it 
neither  altered  the  potential  of  the  iron  nor  caused  the  iron  containing  it  to 
produce  any  unusual  reducing  effect  on  mildly  oxidizing  solutions.  It  is 
probable  that  the  hydrogen  thus  held  by  the  fine  powder  was  merely  adsorbed, 
as  the  gas  is  adsorbed  by  charcoal. 

That  the  gas  could  exist  in  the  metal  in  a  radically  different  condition  we 
did  not  at  first  suspect ;  but  this  conclusion  was  forced  upon  us  by  the  sur- 
prising and  quite  unexpected  results  of  several  other  experiments. 

In  the  first  place,  several  pieces  of  porous  iron  which  had  been  reduced  at 
high  temperature,  slightly  oxidized  by  long  standing  in  the  air,  and  then 
reheated  in  hydrogen  at  575°  gave  at  first  an  extraordinarily  high  value  for 
the  usual  cell  couple,  namely  0.825.  In  seven  hours  this  had  decreased  to 
0.810,  and  in  five  days  became  constant  at  0.798,  essentially  the  normal  value. 


^  Baxter  has  shown  that  pure  iron  reduced  at  a  high  temperature  contains  but  little 
occluded  hydrogen.    Am.  Chem.  Journ.,  22,  363  (1899). 

32 


THE   QUENCHING  OF   IRON.  33 

A  slight  indication  of  an  excessive  value  of  this  kind  had  already  appeared 
in  other  previous  samples,  many  of  which  after  a  short  immersion  in  ferrous 
sulphate  showed  a  slight  maximum.  This  was  noticeable  even  in  samples 
which  had  been  ignited  in  a  vacuum.  In  the  cases  mentioned  at  the  begin- 
ning of  this  paragraph,  after  standing  some  time,  bubbles  of  hydrogen 
appeared  around  the  metal,  as  the  electromotive  force  decreased.  This 
seemed  to  show  that  in  some  way  the  excessive  electromotive  force  was  con- 
nected with  hydrogen.  But  hydrogen  gas  has  a  lower,  not  a  higher  potential 
than  iron.  Therefore,  it  is  clear  that  if  the  abnormality  is  produced  by 
hydrogen,  this  impurity  must  exist  in  the  metal  in  a  different  form — no 
longer  as  merely  adsorbed  gas  on  the  surface  of  the  fine  powder,  but  in  some 
new  state,  inclosed  in  the  less  open  structure  of  the  sintered  iron. 

More  light  was  thrown  upon  all  these  matters  by  interesting  series  of 
experiments  which  showed  that  the  dissolved  active  hydrogen  could  be  more 
easily  introduced  into  the  iron  in  other  ways.  There  follows  a  brief  descrip- 
tion of  these  experiments. 

The  first  series  furnishing  this  further  light  was  a  set  of  experiments 
originally  begun  as  a  preliminary  attempt  to  cause  a  change  in  the  internal 
structure  of  iron  by  quick  cooling.  In  it  iron  was  plunged  while  hot  into 
water. 

In  order  to  quench  iron  suddenly  from  a  high  temperature  without  coating 
the  metal  with  a  hardly  soluble  film  of  oxide,  it  was  clearly  necessary  to 
conduct  in  an  atmosphere  free  from  oxygen  both  the  heating  of  the  metal 
and  the  transference  to  the  cooling  agent. 

An  atmosphere  of  hydrogen  was  first  used  for  this  purpose.  As  before, 
a  stout  tube  of  Berlin  porcelain  was  erected  in  a  vertical  position,  the  middle 
portion  being  heated  by  a  Fletcher  furnace.  Pure  electrolytic  hydrogen, 
thoroughly  washed  and  dried,  was  supplied  at  the  top  of  the  tube  through  a 
cooled  Hempel  stopper,  which  was  further  protected  against  the  rapid  con- 
vection of  hot  hydrogen  by  a  pipe-bowl  suspended  at  the  upper  limit  of  the 
heated  zone  by  means  of  an  iron  wire.  The  iron  was  heated  at  first  in  an 
unglazed  basket  of  the  best  porcelain,  and  in  the  later  experiments  on  a 
shelf  or  disk  of  the  same  material.  Simple  mechanical  devices  as  before 
enabled  the  iron  to  be  plunged  or  dropped  quickly  into  the  cooling  agent  at 
the  base  of  the  tube.  The  temperature  of  heating. was  determined  by  a 
Le  Chatelier  platinum-rhodium  thermoelectric  junction.  Cold  boiled  water 
was  used  as  the  cooling  agent  at  the  base  of  the  tube,  the  operation  being, 
therefore,  merely  the  quenching  of  pure  iron  without  exposure  to  oxygen  gas. 

Even  the  first  experiments  gave  interesting  results.  Porous  iron  reduced 
at  800°  was  quenched  from  1,000*^.     Twenty  minutes  afterwards  the  super- 


34  ELECTROMOTIVE    FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 

ficially  oxidized  sample  was  immersed  in  ferrous  sulphate  and  half  an  hour 
later  it  gave  the  value  0.89  volt  against  the  decinormal  electrode,  instead  of 
only  0.79  as  before.  This  extraordinary  value  was  only  temporary,  the 
electromotive  force  falling  rapidly,  at  the  end  of  one  hour  being  0.825  volt 
and  of  five  hours,  0.803.  Three  other  pieces  quenched  at  the  same  time  gave 
similar  results.  Because  the  electromotive  force  fell  rapidly  it  appeared  pos- 
sible that  an  immediate  reading  might  have  given  even  higher  results.  A 
repetition  of  the  experiment  with  frequent  readings  was  evidently  very 
desirable. 

The  second  quenching  was  a  repetition  of  the  first.  To  remove  the 
adhering  water  after  the  operation,  the  samples  of  iron  were  rinsed  for  a 
minute  or  two  in  ferrous  sulphate  solution  before  putting  into  the  cells,  thus 
preventing  dilution  of  the  normal  solution  in  which  they  were  measured. 
The  results  verified  those  of  the  first  quenching,  but  added  to  them,  in  that 
the  later  ones  showed  the  speed  of  decrease  in  potential  to  be  much  less  in 
the  first  half  hour  than  afterwards.  The  potential  started  at  about  0.9  volt, 
and  after  remaining  almost  constant  for  thirty  minutes  began  to  fall  off 
rapidly.  A  number  of  other  similar  quenchings  were  made,  most  of  which 
gave  similar  results,  only  a  few  showing  no  excess  of  potential.  Typical 
examples  are  given  in  the  following  table,  under  the  heading.  Sample  44. 

Further  experiments  were  made  to  determine  if  the  temperature  before 
quenching  caused  any  considerable  effect  on  the  results. 

The  apparatus  used  was  the  same  as  the  one  which  served  in  the  previous 
experiments.  One  sample  was  quenched  from  695°  and  another  from  600°. 
In  both  cases  the  quenched  iron  rose  to  a  maximum  electromotive  force  less 
strikingly  above  the  normal  value,  the  former  attaining  0.87  volt  and  the 
latter  0.82  volt,  which  potentials,  as  usual,  settled  down  to  the  normal  values 
of  0.795  in  the  course  of  half  a  day.  These  experiments  showed  that  the 
excessive  value  of  the  electromotive  force  was  directly  dependent  upon  the 
temperature  before  quenching,  a  higher  temperature  giving  a  greater  excess. 
They  added  to  the  evidence  already  given  that  the  "  gamma  "  or  high-tem- 
perature phase  of  iron  could  have  nothing  to  do  with  the  excessive  values, 
because  the  latest  quenching  occurred  from  a  temperature  below  the  recog- 
nized transition  point.  It  will  be  remembered  in  this  connection  that  pure 
iron  quenched  from  1,100°  on  a  cold  iron  plate  gave  normal  values  for  its 
electromotive  force. 

The  experiment  was  next  made  of  reheating  the  quenched  iron  in  hydro- 
gen in  order  to  discover  whether  or  not  it  would  return  to  its  normal  condi- 
tion. Two  pieces  of  this  same  quenched  iron  were  thoroughly  dried  in 
alcohol  and  ether  and  three  days  later  were  reheated  in  hydrogen  for  one 
hour  (the  highest  temperature  being  faint  redness)  and  then  slowly  cooled. 


THE   SOURCE   OF   THE   OCCLUDED   HYDROGEN. 


35 


The  results,  together  with  those  of  the  previously  described  quenched  iron, 
are  given  in  the  following  table : 


Table  8. — The  effect  of  reheating  quenched  iron. 


Time  elapsed 

after 

inijnersion. 

Sample  44, 
quenched,  not  reheated. 

Sample  46, 

reheated 

iron. 

Sample  46, 

reheated 

iron. 

Fresh. 

Exposed  to  air. 

Volt. 

Volt. 

Volt. 

Volt. 

Oh      3m 

0.880 

0.79 

0.796 

0.794 

0    11 

0.910 

0.80 

0.800 

0,801 

0    20 

0.906 

0.90 

0.800 

0.804 

0    45 

0.890 

0.89 

0.797 

0.804 

2 

0.810 

0.81 

0.797 

0.803 

4 

0.797 

0.80 

0.797 

0.802 

It  is  clear  at  once  that  samples  45  and  46  had  once  more  acquired  the 
same  condition  they  possessed  before  they  were  quenched — as  was  to  be 
expected.  Thus  there  can  be  no  doubt  that  the  reheated  iron  returns  essen- 
tially to  its  normal  condition. 

From  these  experiments  it  was  clear  that  something,  either  chemical  or 
physical,  had  happened  to  the  iron  during  its  heating  and  plunge  into  water 
which  caused  it  to  assume  an  excessive  potential.  Two  possible  causes 
might  underlie  these  interesting  results.  Either  the  sudden  cooling  might 
have  preserved  the  iron  in  an  unstable  allotropic  form  yielding  a  higher 
electromotive  force,  or  else  occluded  hydrogen  might  in  some  way  be  respon- 
sible for  the  differences.  The  former  of  these  explanations  was  disproved 
by  the  already  described  experiments  on  cooling  iron  in  a  vacuum  on  a  cold 
iron  plate.  Hence  only  the  latter  explanation  remained,  and  the  phenomenon 
was  obviously  to  be  classed  with  the  occlusion  of  hydrogen  from  the  hot  gas. 

It  now  became  an  interesting  matter  to  find  whether  this  hydrogen  came 
from  the  hydrogen  gas  or  from  the  water  during  the  instant  of  quenching. 
This  was  easily  decided  by  an  experiment  in  which  the  metal  was  heated  in 
and  quenched  from  an  atmosphere  of  nitrogen.  The  quenching  apparatus 
was  exactly  the  same  as  that  used  in  all  the  previous  experiments.  The 
nitrogen  was  prepared  by  Wanklyn's  method  of  blowing  air  through  aqua 
ammonia,  passing  this  ammoniacal  air  over  hot  copper  gauze,  and  purifying 
the  resulting  nitrogen  in  the  usual  way.  All  rubber  used  in  the  apparatus 
had  been  boiled  in  caustic  solution  and  was  thickly  coated  on  the  inside  with 
the  semi-solid  paraffin.  In  this  apparatus  a  typical  sample  of  porous  iron 
was  heated  for  twelve  minutes  in  nitrogen  at  a  temperature  of  1,040°  and 
was  then  quenched  in  ice-water.     Of  the  two  pieces  thus  quenched,  one  was 


36  ELECTROMOTIVE    FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 

immersed  in  the  ferrous  sulphate  solution  five  minutes  after  quenching, 
while  the  other  was  thoroughly  dried  in  alcohol  and  ether  and  put  into  the 
cell  to  be  measured  on  the  following  day. 

The  results  agreed  in  a  general  way  with  those  already  obtained  by 
quenching  from  an  atmosphere  of  hydrogen.  They  differed  in  two  respects. 
The  maximum  electromotive  force  was  0.03  volt  higher  than  any  other  thus 
far  obtained,  rising  to  0.94  volt,  and  this  high  electromotive  force  was 
retained  for  a  much  longer  period  of  time.  These  are  differences  in  degree 
only,  and  since  the  other  results  differed  among  themselves,  although  to  a 
less  extent,  the  higher  value  is  to  be  ascribed  merely  to  more  favorable 
experimental  conditions. 

Could  the  formation  of  iron  nitride  in  any  way  have  produced  the  high 
potential  in  this  case?  Reflection  promptly  decided  this  question  in  the 
negative.  Iron  nitride  is  formed  only  at  low  temperatures  and  is  decom- 
posed even  in  a  stream  of  ammonia  at  higher  temperatures ;  "  free  nitrogen 
is  not  able  to  unite  with  iron,  even  when  the  latter  is  reduced  in  nitrogen, 
and  the  formation  of  nitride  is  dependent  on  the  coming  in  contact  of  nascent 
nitrogen  with  iron."  ^  Moreover,  the  nitride  begins  to  decompose  in  a 
stream  of  nitrogen  at  600°,  and  the  nitride  FcgN  is  an  exothermic  compound. 
Hence  probably  it  has  a  lower  electromotive  force  than  iron,^"  not  a  higher 
one,  and  the  nitride  is  evidently  out  of  the  question. 

Upon  standing  in  ferrous  solution  the  quenched  iron  evolved  an  appre- 
ciable volume  of  gas,  which  was  shown  to  be  hydrogen  by  explosion  with 
oxygen  in  a  micro-eudiometer. 

In  view  of  this  fact,  and  because  iron  quenched  from  nitrogen  gave  the 
same  high  potential  as  iron  quenched  from  hydrogen,  there  can  be  no  doubt 
that  the  bulk  of  the  occluded  hydrogen  must  have  come  from  the  water  at 
the  moment  of  quenching.  Some  of  the  iron  must  have  been  oxidized,  and 
the  resulting  nascent  hydrogen  must  have  dissolved  in  the  iron,  in  its  active 
state.  No  other  explanation  seemed  to  be  compatible  with  the  extraordinary 
potential  observed. 

Other  workers  on  this  subject  (for  example  Heyn,^  who  found  that 
hydrogen  exists  in  iron  quenched  from  above  730°  in  an  atmosphere  of 
hydrogen)  have  usually  supposed  that  the  occluded  impurity  was  taken 
from  the  gas  and  not  from  the  water,  but  our  experiments  show  con- 
clusively that  the  water  is  the  source  of  the  greater  part  of  the  hydrogen. 


«« Stahlschmidt,  Fogg.  Ann.,  125,  37  (1865). 

**  Assuming  total  energy  change  to  be  an  approximate  guide  to  free  energy  change 
in  reactions  of  this  kind. 

'"Stahl  und  Eisen,  20,  837  (1900).  See  also  Roberts-Austen,  Fifth  report  of  the 
Alloys  Research  Committee,  Proc.  Inst.  Mech.  Eng.,  1899,  p.  35. 


THE   ELECTROMOTIVE   FORCE  OF   OCCLUDED   HYDROGEN.  3/ 

The  discovery  that  in  this  case  nascent  hydrogen  may  be  absorbed  more 
freely  than  the  gaseous  material  by  iron  at  once  reminded  us  of  the  well- 
known  absorption  of  the  light  element  by  metals  used  as  a  cathode  in  acid 
solutions.  Cailletet*'  and  Johnson*^  showed  in  1875  that  iron  will  take  up 
at  ordinary  temperatures  electrolytic  hydrogen  deposited  upon  it,  while 
ordinary  gaseous  hydrogen  has  no  effect.  Johnson's  work  was  particularly 
interesting.  His  iron  was  in  the  form  of  soft  wire  and  his  simple  method  of 
testing  for  hydrogen  was  to  bend  the  wire  to  test  its  brittleness,  since  he  had 
discovered  the  interesting  fact  that  hydrogen  imparted  this  property  to  iron 
previously  tough.  Immersing  the  wire  in  acids  which  react  on  it  to  form 
hydrogen  uniformly  made  it  brittle.  Also  when  the  fractured  surface  was 
wetted  with  water  it  was  seen  to  froth.  Making  the  wire  the  cathode  in 
neutral,  acid,  or  alkaline  solutions  had  the  same  effect,  but  iron  anodes  were 
unaffected  even  in  the  acid  solution.  Other  wires  were  placed  in  a  bottle 
full  of  water  and  hydrogen  was  made  to  bubble  violently  through  the  water, 
without  any  trace  of  absorption. 

Another  interesting  research  yielding  similar  results  was  conducted  by 
Bellati  and  Lussana.^  A  barometer  was  closed  at  the  top  by  an  iron  plate, 
and  by  cementing  a  glass  ring  on  this  an  electrolytic  cell  was  made  in  which 
hydrogen  was  generated  at  the  iron  plate ;  the  mercury  at  once  fell  by  dif- 
fusion of  the  hydrogen  through  the  plate  into  the  barometric  vacuum.  This 
work  was  later  confirmed  and  amplified  by  Shields.'' 

In  view  of  these  phenomena,  it  became  a  very  interesting  point  to  dis- 
cover if  the  active  hydrogen  introduced  into  cathode  iron  from  solution  is 
capable  of  raising  the  potential  of  a  normal  cell  to  the  value  of  0.93,  that 
observed  with  spongy  iron  quenched  from  1,100°  in  nitrogen.  Shields  had 
already  observed  that  this  occluded  hydrogen  raised  the  electromotive  force 
of  the  iron  above  the  normal  value,  but  the  maximum  had  not  been 
determined. 

This  point  was  easily  tested.  In  order  to  obtain  satisfactory  evidence  it 
is  of  course  necessary  that  iron  alone  should  be  immersed  as  cathode;  the 
platinum  wire  supporting  it  must  be  above  the  liquid.  Preliminary  experi- 
ments showed  that  a  large  excess  of  potential  is  as  a  matter  of  fact  attained, 
but  that  it  falls  off  with  very  great  rapidity.     Spongy  iron  which  had  been 


^"Comptes  rendus,  80,  319  (1875). 

''Proc.  Roy  Soc.   (London),  23,  168  (1875). 

^^  Bellati  and  Lussana,  Z.  Phys.  Chem.,  7,  229  (1891). 

'"Shields,  Chem.  News,  65,  195  (1892).  Further  discussion  of  this  matter  will  be 
found  in  papers  mentioned  later;  see  also  Hoitsema,  Z.  Phys.  Chem.,  17,  i  (1895); 
Winkelmann,  Drude's  Ann.,  8,  388  (1902)  ;  Richardson,  Nicholl  and  Parnell,  Phil. 
Mag.  (6),  8,  I  (1904).  St.  Schmidt  considers  the  assumption  of  a  split  of  the  hydro- 
gen molecule  to  be  unnecessary  and  unwarranted.     (Drude's  Ann.,  13,  747  (1904).) 


38  ELECTROMOTIVE   FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 

made  the  cathode  of  a  dense  current  in  an  acid  sohitlon  for  an  hour  was 
quickly  washed  with  neutral  ferrous  sulphate,  and  was  measured  as  soon  as 
possible.  In  half  a  minute  after  immersion  it  had  sunk  to  0.822  volt,  in  four 
minutes  later  0.800,  and  in  an  hour  to  constancy  at  0.790.  Evidently  the 
hydrogen  is  only  very  superficially  deposited,  and  most  of  it  is  gone  before 
measurement  is  possible.  Longer  exposure  as  cathode  might  have  more 
effect.  Much  more  striking  results  were  obtained  from  electrolytic  iron 
obtained  from  a  neutral  solution  with  the  help  of  a  current  so  dense  as  to 
deposit  hydrogen  with  the  iron.  Such  a  specimen  (sample  48),  measured 
about  three  seconds  after  the  breaking  of  the  current,  gave  a  potential  of  0.93, 
equal  to  that  observed  in  the  quenched  iron.  In  a  minute  this  had  sunk  to 
0.85  volt ;  but  even  after  an  hour  it  remained  as  high  as  0.83  volt,  not  sinking 
to  about  the  normal  value  0.798  for  several  days. 

Thus  it  is  clear  that  as  far  as  potential  measurements  can  show,  the 
hydrogen  driven  into  iron  with  the  help  of  electrical  energy  is  essentially 
similar  to  that  absorbed  in  the  act  of  quenching,  although  the  latter  operation 
seems  to  be  especially  favorable  to  the  occlusion.  Probably  the  action  of 
the  red-hot  core  upon  the  steam  produced  by  the  exterior  of  the  porous  metal 
supplies  the  nascent  hydrogen  to  the  iron  at  a  temperature  especially  suitable 
for  occlusion. 

It  is  interesting  to  note  that  even  without  outside  electrical  assistance 
hydrogen  in  this  active  form  may  be  taken  up  by  iron  from  acid  solutions. 
Upon  immersing  the  metal  in  an  acid  solution  of  ferrous  sulphate,  hydrogen 
is  of  course  evolved,  and  the  potential  of  the  iron  while  still  in  the  acid  is 
lowered  as  much  as  two  decivolts — evidently  by  the  coating  of  gas,  for 
hydrogen  gas  has  a  potential  much  lower  than  iron.  But  besides  this  super- 
ficial eflfect,  a  more  deep-seated  one  is  occurring,  as  is  easily  shown. 

Upon  removing  this  iron  quickly  to  a  neutral  solution  of  ferrous  sulphate 
and  measuring  at  once,  its  potential  is  found  to  be  above  the  normal  value, 
the  usual  cell  often  reading  as  high  as  0.83  instead  of  0.78  or  0.79.  In  a 
few  hours  it  settles  down  as  usual  to  its  normal  level,  evolving  in  the  process 
bubbles  of  hydrogen  gas.  The  iron  alone  could  not  of  course  raise  the  active 
hydrogen  which  it  absorbs  to  a  concentration  above  that  capable  of  giving 
an  electromotive  force  equal  to  its  own  (0.79  with  the  decinormal  electrode), 
but  with  the  help  of  the  osmotic  pressure  of  the  ionized  hydrogen  in  the 
acid  more  hydrogen  is  driven  in.  This  excess  manifests  its  potential  and 
changes  to  hydrogen  gas  when  the  surrounding  acid  is  removed.  As  a 
change  of  only  tenfold  in  the  concentration  of  the  ionized  hydrogen  would 
be  expected  to  produce  a  change  of  potential  of  nearly  0.06  volt,  the  observed 
effect  is  by  no  means  excessively  large.     Stated  in  another  way,  it  may  be 


THE   DISCHARGE   OF   THE   EXCESSIVE   POTENTIAL.  39 

said  that  the  hydrogen  taken  in  by  iron  from  an  acid  solution  is  in  equilibrium 
with  nascent  hydrogen,  and  therefore  possesses  a  high  chemical  potential. 

In  the  course  of  the  quenching  experiments  it  was  found  that  iron 
charged  with  active  hydrogen  lost  its  impurity  much  more  quickly  when 
immersed  in  ferrous  sulphate  than  when  immersed  in  water  or  kept  in  air. 
This  interesting  fact  seemed  worthy  of  more  careful  study,  because  it  might 
be  capable  of  throwing  light  on  the  singular  occlusion.  Accordingly  the 
following  series  of  experiments  was  instituted  upon  six  pieces  of  iron,  all 
quenched  at  once  under  such  conditions  that  all  were  exactly  alike : 

Piece  No.  51  was  immersed  in  ferrous  sulphate  and  measured  immediately 
after  quenching.  Its  electromotive  force  began  at  about  0.8  volt,  gave  the 
maximum  value  0.91  volt  after  a  few  minutes,  and  then  fell  off  as  before, 
reaching  0.90  volt  in  half  an  hour  and  0.793  volt  in  four  hours. 

No.  52  was  left  in  the  water  in  which  it  was  quenched  for  an  hour  and 
twenty  minutes  before  measurement.  When  the  electromotive  force  had 
reached  its  maximum  of  0.90  volt  after  26  minutes  subsequent  immersion 
in  ferrous  sulphate.  No.  51  had  passed  that  point  over  an  hour  and  a  half 
before. 

No.  53  was  also  left  in  water  and  was  first  measured  after  two  hours  and 
forty  minutes  had  elapsed.  By  the  time  it  gave  its  highest  electromotive 
force,  0.91  volt.  No.  i  had  dropped  to  0.803  volt. 

No.  54  was  left  in  water  for  26  hours  and  then  immersed  in  ferrous 
sulphate.  It  behaved  but  little  differently  from  No.  53,  but  its  maximum  was 
lower,  being  only  0.85  volt.  Before  its  immersion  all  the  preceding  had 
reached  their  constant  values  0.795. 

No.  55  was  thoroughly  dried  in  alcohol  and  ether  immediately  after 
quenching  and  kept  in  a  desiccator  an  equal  length  of  time  (26  hours) 
before  measurement.  This  specimen  began  at  0.79  volt,  rose  regularly  to 
0.88  volt  in  two  hours,  and  reached  0.80  volt  again  in  twenty-four  hours. 

No.  56  also  was  immediately  thoroughly  dried  and  was  measured  after 
73  hours,  giving  a  similar  but  less  marked  maximum  (0.85  volt). 

The  whole  series  of  experiments  was  then  repeated  with  essentially  identi- 
cal results. 

The  comparison  of  the  results  of  these  series  of  experiments  shows  that 
mere  exposure  to  dry  air  slowly  lowers  the  high  -electromotive  force  due  to 
quenching,  that  pure  water  hastens  this  lowering  a  very  little  more  than 
exposure  to  the  air,  but  that  immersion  in  ferrous  sulphate  quickly  estab- 
lishes equilibrium.  The  more  typical  of  these  experiments,  with  some  of  the 
previous  ones,  are  plotted  in  figure  6. 

It  became  now  an  interesting  matter  to  discover  if  any  other  electrolyte 


40 


ELECTROMOTIVE    FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 


would  effect  the  adjustment  of  equilibrium  as  quickly  as  the  ferrous  sul- 
phate. In  an  easily  carried  out  variation  on  the  preceding  series,  a  normal 
solution  of  potassic  suphate  was  used  as  a  typical  electrolyte.  Brief  im- 
mersion in  this  solution  seemed  to  show  no  more  effect  in  discharging  the 
hydrogen  than  in  water,  but  long  immersion  showed  a  slight  difference  in 
favor  of  the  electrolyte.  The  rate  was  nevertheless  so  much  slower  than 
that  observed  in  the  case  of  ferrous  sulphate  as  to  indicate  entirely  another 
mechanism  of  reaction. 


0.9. 


0.8 


0.7 


0.6 


as 


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10     II      12     13     14    15     16     17     18     19     20 


Fig.  6. — ^The  Changing  Potential  of  the  Iron  Electrode. 

In  the  direction  of  ordinates  are  plotted  the  potentials  of  the  iron  electrodes  in  normal  sulphate,  in 
combination  with  a  decinormal  calomel  electrode.  Time  in  hours  is  plotted  in  the  direction  of 
abscissae.  The  curves  are  typical,  No.  7  of  porous  iron  which  has  been  exposed  to  air,  No.  33  of 
pure  fused  iron,  No.  48  of  porous  iron  which  has  been  made  a  cathode.  No.  51  of  porous  iron 
quenched  in  water,  and  No.  53  of  the  same  after  exposure  to  air  for  2%  hours. 


The  progress  of  the  evolution  of  the  gas  in  ferrous  sulphate  may  be  sup- 
posed to  be  as  follows :  The  active  hydrogen,  having  a  higher  potential  than 
the  iron,  causes  metal  to  deposit  from  the  ferrous  sulphate,  and  the  acid 
thus  formed  immediately  attacks  the  deposit,  or  a  neighboring  portion  of 
metal.  Without  the  interposition  of  the  iron  as  carrier,  the  hydrogen  seems 
to  find  it  difficult  to  go  from  its  occluded  to  its  gaseous  state.  When  an 
electrolyte  which  contains  no  easily  deionized  cation  is  substituted  for  the 
ferrous  sulphate,  the  reaction  involving  deposition  can  not  take  place,  and  the 
hydrogen  remains  in  its  occluded  condition. 


THE    POSSIBLE    EXPLANATION.  4I 

These  considerations  show  further  that  the  final  potential  which  the  iron 
attains  after  long  immersion  in  ferrous  sulphate  is  probably  the  true  poten- 
tial, because,  since  both  iron  and  ferrous  sulphate  are  greatly  in  excess,  the 
equilibrium  finally  attained  must  correspond  to  these  two  alone.  Even  if 
some  dissolved  hydrogen  is  retained  by  the  iron  it  can  not  possess  a  perma- 
nent potential  above  that  of  the  iron.  That  this  is  true  is  shown  by  the 
fact  that  quenched  iron  finally  attains  a  potential  equal  to  that  finally 
attained  by  a  similar  sample  which  has  been  long  exposed  to  the  air — one 
approaching  this  value  from  above,  the  other  from  below. 

The  attempt  to  explain  the  nature  and  condition  of  the  active  hydrogen  in 
iron  is  assisted  by  a  suggestive  paragraph  which  occurs  in  a  paper  by  Thos. 
Graham  *"  '^  On  the  relation  of  hydrogen  to  palladium  and  on  hydro- 
genium  " : 

The  chemical  properties  of  hydrogenium  *^  also  distinguish  it  from  ordinary  hydro- 
gen. The  palladium  alloy  precipitates  mercury  and  calomel  from  a  solution  of  the 
chloride  of  mercury  without  any  disengagement  of  hydrogen;  that  is,  hydrogenium 
decomposes  chloride  of  mercury,  while  hydrogen  does  not.  .  .  .  Hydrogenium 
(associated  with  palladium)  unites  with  chlorine  and  iodine  in  the  dark,  converts  red 
prussiate  of  potash  into  yellow  prussiate,  and  has  considerable  deoxidizing  powers.  It 
appears  to  be  the  active  form  of  hydrogen  as  ozone  is  of  oxygen. 

Of  these  chemical  tests  one  was  easily  extended  to  hydrogen  contained 
in  iron.  It  was  found  that  carefully  cleaned  pure  iron  wire,  and  also  porous 
iron  which  had  been  kept  for  a  year  in  the  pure  air  of  a  desiccator  over 
potash,  had  no  appreciable  tendency  to  reduce  neutral  potassic  ferricyanide. 
Even  after  the  iron  had  stood  in  a  solution  of  this  salt  for  five  days  the 
solution  gave  no  precipitate  with  ferric  chloride.  Such  iron  was  evidently 
practically  free  from  active  hydrogen.  On  the  other  hand,  iron  which  had 
been  freshly  made  showed  a  marked  reducing  tendency,  and  this  tendency 
was  even  greater  in  quenched  iron  and  iron  which  had  been  used  as  a 
cathode  (or  had  been  simply  immersed  in  acid)  and  thoroughly  washed. 
These  results  are  quite  in  accord  with  the  potential  measurements  already 
recorded,  and  furnish  additional  evidences  of  active  hydrogen  in  freshly 
reduced  or  quenched  iron,  but  point  to  its  absence  on  the  surface  of  iron 
long  exposed  to  the  air. 

In  what  form  may  this  hydrogen  be  supposed  to  exist?  Ramsay  was  per- 
haps the  first  to  suggest  that  hydrogen  undergoes  dissociation  when  passing 
through  a  hot  metal,  or  through  one  made  a  cathode  in  acid  solution.** 
Our  experience  supports  this  hypothesis,  and  seems  to  be  explicable  in  no 


**Proc.  Roy.  Soc.    (London),  17,  219   (1869);  Collected  Papers,  p.  290-299;   Pogg. 
Ann.,  138,  49  (1869). 

*^  Hydrogenium  =  hydrogen  in  a  metal. 

"Sir  William  Ramsay,  Phil.  Mag.,  5,  38,  206  (1894). 


42  ELECTROMOTIVE    FORCE   OF    IRON    AND   OCCLUDED    HYDROGEN. 

Other  way.  In  one  respect  only  does  Ramsay's  opinion  appear  question- 
able, namely,  in  his  assumption  that  the  dissociated  gas  is  ionized/'  In 
recent  years  the  word  ion  has  been  used  in  so  many  senses  that  an  unfortu- 
nate vagueness  has  crept  into  its  definition ;  but  the  idea  of  ionization  seems 
always  at  least  to  be  associated  with  electric  charges.  Now,  in  this  case 
there  seems  to  be  no  need  of  assumption  of  an  electric  charge  on  the 
occluded  hydrogen  atom ;  indeed  it  is  hard  to  see  how  much  a  charge  could 
be  held  in  the  midst  of  so  good  a  conductor  as  iron.  To  the  writers  it 
appears  much  more  probable  that  the  hydrogen  is  rather  in  the  condition  of 
nascent  hydrogen,  set  free  from  the  positive  charges  which  had  caused  it  ta 
ionize  in  the  aqueous  solution,  but  not  yet  consolidated  into  the  form  of 
hydrogen  gas.  It  would  appear  that  iron  is  permeated  with  minute  cavities 
into  which  only  this  dissociated  form  of  hydrogen  is  able  to  enter,  and  that 
upon  all  occasions  when  the  nascent  element  is  liberated  in  the  presence  of 
the  iron,  the  opportunity  of  entrance  is  at  once  seized.  In  the  wording  of 
the  atomic  hypothesis,  the  active  hydrogen  occluded  by  iron  seems  to  be 
atomic  but  not  ionized  hydrogen,  very  different  in  its  properties  from  the 
molecular  hydrogen  which  is  adsorbed  by  the  fine  powder  reduuced  at  low 
temperatures. 

In  conclusion,  it  is  a  pleasure  to  express  our  gratitude  to  the  Carnegie 
Institution  of  Washington  for  generous  pecuniary  assistance  in  this 
investigation. 

SUMMARY  OF  PART  FIRST. 

( 1 )  A  method,  both  rapid  and  convenient,  is  given  for  the  preparation  of 
iron  containing  no  impurity  but  hydrogen. 

(2)  The  potential  of  this  spongy  iron  in  ferrous  sulphate  was  measured 
and  found  to  be  at  first  greatly  affected  by  previous  exposure  to  the  air. 
After  long  immersion  in  ferrous  sulphate  solution  a  constant  and  trustworthy 
value  was  reached. 

(3)  Even  the  enormous  pressure  of  about  350,000  kilograms  per  square 
centimeter  did  not  produce  any  appreciable  permanent  change  in  this  value, 
although  the  masses  were  effectively  cold- welded.  Taken  in  conjunction 
with  the  results  of  Spring,  this  fact  is  shown  to  be  consistent  with  the 
hypothesis  of  compressible  atoms. 

(4)  Measurements  of  the  free-energy  change  in  iron  during  a  pull  upon 
a  wire  great  enough  to  cause  rupture  gave  results  showing  that  this  change 
must  be  very  small,  and  called  attention  to  regrettable  errors  in  previous 
work  on  this  subject. 


*' Ramsay,  "Modern  Chemistry,"  II,  31  (London,  Dent,  1904). 


SUMMARY   AND   CONCLUSION.  43 

(5)  Iron  reduced  at  a  low  temperature  was  found  to  have  an  electro- 
motive force  higher  by  at  least  0.02  volt  than  iron  which  had  been  fused. 
If  the  normal  calomel  electrode  is  taken  as  having  a  single  potential  differ- 
ence of  0.56,  pure  compact  iron  has  a  single  potential  difference  of  0.15, 
and  spongy  or  porous  iron  about  0.17  to  0.18  volt.  The  former  is  to  be 
chosen  as  the  normal  value.  It  is  pointed  out  that  the  much  lower  results 
of  others  are  probably  to  be  referred  to  the  acidity  of  their  solutions  and 
to  insufficient  waiting  for  equilibrium. 

(6)  No  important  change  in  these  values  was  caused  by  sudden  cooling 
from  a  high  temperature. 

(7)  Speculations  concerning  the  relations  of  these  facts  to  the  structure 
and  internal  pressure  and  solubility  of  iron  are  tentatively  recorded.  The 
difference  of  potential  of  the  different  forms  of  iron  is  probably  but  not 
certainly  to  be  referred  to  the  different  sizes  of  their  separate  particles. 

SUMMARY  OF  PART  SECOND. 

(8)  It  was  found  that  hydrogen  could  be  taken  up  by  finely  powdered 
iron  reduced  at  low  temperatures  without  affecting  the  metal's  electromotive 
force.  When  the  metal  is  wholly  coated  with  hydrogen  the  electromotive 
force  is  diminished.  It  is  probable  that  hydrogen  thus  held  is  merely 
adsorbed  or  held  as  molecular  hydrogen. 

(9)  It  was  found  further  that  by  quenching  in  water,  whether  from 
hydrogen  or  from  nitrogen  gas,  that  iron  takes  up  hydrogen  in  an  active 
form,  raising  the  single  potential  by  as  much  as  0.15  volt.  This  hydrogen  is 
quickly  expelled  in  ferrous  sulphate  solution,  and  very  slowly  in  water  or 
potassic  sulphate  solution,  the  potential  returning  to  the  normal  value.  The 
gas  thus  evolved  was  proved  to  be  hydrogen  gas. 

(10)  A  small  amount  of  hydrogen  in  the  same  active  form  may  be  taken 
in  from  hot  hydrogen  gas. 

(11)  Active  hydrogen  thus  occluded  by  iron  seems  to  be  in  every  way 
similar  to  that  occluded  by  iron  in  the  presence  of  nascent  hydrogen,  whether 
this  is  chemically  or  electrolytically  produced. 

(12)  It  is  pointed  out  that  the  most  reasonable  explanation  of  these  facts 
is  to  suppose  that  the  active  dissolved  hydrogen  is  dissociated  but  not  ionized. 

The  investigation  will  be  continued  in  the  near  future. 


p 


p 


^l^IJ™*'^*^****'^^^^ 


mi 


Richards,  T.W, 

The  electromotive  force 
of  iron  under  varying 
conditions,  and  the  effect 
of  occluded  hydrogen. 

PHYSICAL 
SCIENCES 
LIBRARY 


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MMm 


LIBRARY 

UNIVERSITY  OF  CALIFORNIA 

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